High-efficiency yield-increasing exploitation method for natural gas hydrates

Abstract

A high-efficiency yield-increasing exploitation method for natural gas hydrates includes steps of drilling of natural gas hydrate reservoirs along horizontal wells, seepage increasing by fracturing for fracture forming and stability improvement by grouting in the natural gas hydrate reservoirs, and yield improvement by combined exploitation of depressurization of the horizontal wells and heat injection; according to the present invention, drilling time is shortened by rapid drilling along the horizontal wells, the permeability of the reservoirs can be effectively improved by fracturing for fracture forming, the stability of the reservoirs can be improved by injecting foam cement slurry into the reservoirs, and the yield of the natural gas hydrates can be improved by the combined exploitation method of depressurization of the horizontal wells and heat injection.

Claims

1. A high-efficiency yield-increasing exploitation method for natural gas hydrates, comprising the following steps: (i) drilling of natural gas hydrate reservoirs along horizontal wells as for the natural gas hydrate reservoirs, drilling well arrays in an exploratory trench drilling manner along the horizontal wells, each of which comprising three horizontal wells: a first horizontal well, and a second horizontal well and a third horizontal well positioned on two sides of the first horizontal well, and each of the horizontal wells comprising a vertical well section, an inclined well section, and a horizontal well section; (ii) seepage increasing by fracturing for fracture forming and stability improvement by grouting in the natural gas hydrate reservoirs after completing a drilling operation, implementing fracturing for fracture forming at the horizontal well section of the first horizontal well, injecting a seawater fracturing fluid into the first horizontal well, and allowing the seawater fracturing fluid to flow into the natural gas hydrate reservoirs along the horizontal well section, so that fractures are formed in the hydrate reservoirs between the first horizontal well and the second horizontal well, and between the first horizontal well and the third horizontal well, then, injecting foam cement slurry into the horizontal well section of the first horizontal well, allowing the foam cement slurry to be distributed all over the natural gas hydrate reservoirs, and curing for forming, and finally, injecting a resin or oligomer chemical sand control agent into a periphery of the well to reduce or avoid a sand production rate during an exploitation of the natural gas hydrates at a later period; and (iii) yield improvement by combined exploitation of depressurization of the horizontal wells and heat injection injecting seawater into the natural gas hydrate reservoirs along the first horizontal well at a temperature of above 60? C., and meanwhile, reducing a bottom pore pressure of the second horizontal well and the third horizontal well, so as to improve a decomposition rate of the natural gas hydrates in the reservoirs under a collaborative action of the two operations.

2. The high-efficiency yield-increasing exploitation method for the natural gas hydrates according to claim 1, wherein in step (i), an interval among different horizontal wells is determined in the following method: firstly, based on a Darcy's law, a pressure distribution in the natural gas hydrate reservoirs is as follows: $\begin{array}{cc}{P}_2=P_1+\frac{Q.Math.?.Math.L}{K.Math.A}& \left(1\right)\end{array}$ wherein P.sub.1 is a bottom pore pressure of the horizontal wells, MPa; P.sub.2 is a pressure in the natural gas hydrate reservoirs, MPa; Q is a flow in pores of the natural gas hydrate reservoirs, m.sup.3/s; ? is a fluid viscosity in the reservoirs, mPa.Math.s; L is a seepage radius of a fluid in the reservoirs, m; K is an absolute permeability of the natural gas hydrate reservoirs, mD; A is a sectional area of seepage flow of the natural gas hydrate reservoirs, m.sup.2; then, in order to achieve a decomposition of the natural gas hydrates during an exploitation of the natural gas hydrates under reduced pressure, the pressure P.sub.2 in the reservoirs needs to conform to the following criteria: $\begin{array}{cc}{P}_2?P_e=10^6\exp \left(\underset{n=0}{\overset{5}{.Math.}}{a_n\left(T+?T_d\right)}^n\right)& \left(2\right)\end{array}$ wherein $\begin{array}{cc}\{\begin{array}{c}{a}_0=-1.94138504464560?10^5\\ a_1=3.31018213397926?10^3\\ a_2=-2.25540264493806?10^1\\ a_3=7.67559117787059?10^{-2}\\ a_4=-1.30465829788791?10^{-4}\\ a_5=8.86065316687571?10^{-8}\end{array}& \left(3\right)\end{array}$ wherein T is a reservoir temperature, K; ?T.sub.d is a temperature at which a decline in a hydrate equilibrium is caused by a thermodynamic hydrate inhibitor, K; ?T.sub.d is obtained by the following formula through calculation: $\begin{array}{cc}?T_d=?T_{d,r}\frac{\ln \left(1-x\right)}{\ln \left(1-x_r\right)}& \left(4\right)\end{array}$ wherein x is a molar fraction of the thermodynamic hydrate inhibitor in a water phase, which is dimensionless; x.sub.r is a reference molar fraction of the thermodynamic hydrate inhibitor in the water phase, which is dimensionless; ?T.sub.d,r is a temperature at which the decline in the hydrate equilibrium is caused under the molar fraction of the thermodynamic hydrate inhibitor as x.sub.r, K; and an interval (L.sub.1=L) between every two horizontal wells is solved according to formulas (1)-(4).

3. The high-efficiency yield-increasing exploitation method for the natural gas hydrates according to claim 1, wherein in step (ii), during fracturing, a fracturing pressure is larger than a fracture forming pressure of the natural gas hydrate reservoirs.

4. The high-efficiency yield-increasing exploitation method for the natural gas hydrates according to claim 3, wherein in step (ii), a preparation process of the foam cement slurry is as follows: mixing a certain mass of cement with water to form cement slurry, and adding a certain mass of foaming agent and foam stabilizer to the cement slurry with stirring to form foam cement slurry; in the preparation of the foam cement slurry, according to the properties of the required foam cement slurry and a mix proportioning principle thereof, the using amount of the cement is as follows: $\begin{array}{cc}{M}_{sn}=\frac{V_{\mathrm{gfc}}.Math.{?}_{\mathrm{gfc}}}{s_a}& \left(5\right)\end{array}$ wherein M.sub.sn is the using amount of the cement, kg; ?.sub.gfc is the design dry density of the foam cement, kg/m.sup.3; V.sub.gfc is the volume of the dry foam cement, m.sup.3; S.sub.a is a mass coefficient, which is dimensionless, wherein the mass coefficient is 1.2 for ordinary Portland cement, and 1.4 for sulfate cement; water supply volume:
M.sub.w=?.Math.M.sub.sn(6) wherein M.sub.w is the water supply volume; a is a basic ratio of water to materials, which is dimensionless; the using amount of the foaming agent: $\begin{array}{cc}{M}_p=\frac{V_{\mathrm{py}}.Math.{?}_{\mathrm{py}}}{b+1}& \left(7\right)\end{array}$ wherein M.sub.p is the mass of the foaming agent in the foam cement slurry, kg; V.sub.py is the volume of a foam concentrate formed from the foaming agent, m.sup.3; ?.sub.py is the density of the foam concentrate formed from the foaming agent, kg/m.sup.3; b is the times of foaming of the foaming agent, which is dimensionless; and the using amount of the foam stabilizer is half of that of the foaming agent.

5. The high-efficiency yield-increasing exploitation method for the natural gas hydrates according to claim 4, wherein in step (ii), an injection rate of the foam cement slurry is as follows:
V.sub.fc?2L.sub.1.Math.L.sub.2.Math.H.Math.S.sub.f(8) wherein V.sub.fc is a volume of the foam cement slurry to be injected into the natural gas hydrate reservoirs, m.sup.3; L.sub.2 is a length of the horizontal well section, m; H is a thickness of the natural gas hydrate reservoirs, m; and S.sub.f is a porosity of the natural gas hydrate reservoirs subjected to fracturing for fracture forming, which is dimensionless.

6. The high-efficiency yield-increasing exploitation method for the natural gas hydrates according to claim 5, wherein in step (ii), during the injection of the foam cement slurry into the horizontal well section of the first horizontal well, when the foam cement slurry is found in the second horizontal well and the third horizontal well, the injection of the foam cement slurry should be stopped, and meanwhile, the foam cement slurry is removed rapidly from the second horizontal well and the third horizontal well; and then, each of the horizontal wells is shut in for 48 h, until the foam cement slurry injected into the natural gas hydrate reservoirs is cured for forming.

7. The high-efficiency yield-increasing exploitation method for the natural gas hydrates according to claim 6, wherein in step (iii), the pressure distributed in the reservoirs is reduced by controlling a pressure decay amplitude at the bottoms of the second horizontal well and the third horizontal well, so that the natural gas hydrates in the reservoirs are decomposed into gases and water, and meanwhile, and the gases flow into the second horizontal well and the third horizontal well under a differential pressure for recovery; and a relationship between absorption of heat from the decomposition of the hydrates and a heat transfer around the reservoirs is coordinated by controlling the bottom pore pressure of the second horizontal well and the third horizontal well with a multi-stage step-by-step depressurization strategy, that is, the pressure decay amplitude at the bottoms of the wells is reduced after the bottom pore pressure is reduced to a hydrate phase equilibrium condition in decomposition, and with every 0.5 MPa of the bottom pore pressure declining, a bottom pore pressure value is maintained until a gas recovery rate declines significantly before the depressurization of the next step.

8. The high-efficiency yield-increasing exploitation method for the natural gas hydrates according to claim 7, wherein the pressure decay amplitude at the bottoms of the wells ranges from 0.1 to 0.2 Mpa/h, and significant reduction in gas recovery rate refers to reduction of gas recovery rate to 1000 cubic meters/day below.

9. The high-efficiency yield-increasing exploitation method for the natural gas hydrates according to claim 4, wherein the foaming agent is preferably sodium dodecyl sulfate, and the foam stabilizer is preferably laurinol.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram of a horizontal well of natural gas hydrate reservoirs according to the present invention; and

(2) FIG. 2 is a schematic top view of an exploitation of natural gas hydrate reservoirs.

(3) Illustrations of all the numerals: 1-Shallow stratum, 2-Natural gas hydrate reservoir, 3-Vertical well section, 4-Inclined well section, 5-Horizontal well section, 6-Horizontal well, 7-Fracture, 8-Second horizontal well, and 9-Third horizontal well.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(4) To make a person skilled in the art better understand the technical solutions of the present specification, the following describes the technical solutions of the present specification clearly and completely, but the present invention is not limited thereto. Any content not described in details in the present invention belongs to the conventional technology in the art.

Example 1

(5) A high-efficiency yield-increasing exploitation method for natural gas hydrates includes the following steps:

(6) drilling of natural gas hydrate reservoirs along horizontal wells

(7) In view of an upper shallow stratum 1 above the deep-water natural gas hydrate reservoirs, featuring softness, and looseness, as for the natural gas hydrate reservoirs 2, drilling well arrays are drilled in an exploratory trench cycle drilling manner, each of which includes three horizontal wells: a first horizontal well 6, and a second horizontal well 8 and a third horizontal well 9 positioned on two sides of the first horizontal well 6, and each of the horizontal wells includes a vertical well section 3, an inclined well section 4, and a horizontal well section 5;

(8) by the exploratory trench drilling manner, the drilling efficiency of single wells may be improved, and the drilling operation along the horizontal wells may increase a contact area between an exploitation wellhole and the reservoirs to improve the exploitation yield of the natural gas hydrates. (1) seepage increasing by fracturing for fracture forming and stability improvement by grouting in the natural gas hydrate reservoirs
After the drilling operation is completed, fracturing is implemented for fracture forming at the horizontal well section 5 of the first horizontal well 6, a seawater fracturing fluid is injected into the first horizontal well and flows into the natural gas hydrate reservoirs along the horizontal well section, so that fractures 7 are formed in the hydrate reservoirs between the first horizontal well and the second horizontal well, and between the first horizontal well and the third horizontal well to improve the permeability of such reservoirs,
then, foam cement slurry is injected into the horizontal well section of the first horizontal well, is distributed all over the natural gas hydrate reservoirs, and is cured for forming for a period of time, thereby improving the stability of the hydrate reservoirs,
and finally, a resin or oligomer chemical sand control agent which is commonly used in the art is injected into a periphery of the well to reduce or avoid a sand production rate during an exploitation of the natural gas hydrates at the later period and improve the safety of the natural gas hydrates in exploitation;
and in this step, the first horizontal well is designed as a fracturing well; the horizontal well section of the first horizontal well is fractured only in order to maintain the stability of the wellhole; seawater which is convenient to obtain, sufficient in supply, and low in costs serves as the fracturing fluid, with a fracturing pressure thereof being larger than a fracture forming pressure of the hydrate reservoirs, and the seawater fracturing fluid flows into the hydrate reservoirs along the horizontal well section of the first horizontal well, forming the fractures in the hydrate reservoirs between the first horizontal well and the second horizontal well and between the first horizontal well and the third horizontal well to improve the permeability of such reservoirs. (2) Yield improvement by combined exploitation of depressurization of the horizontal well and heat injection
The exploitation yield of the natural gas hydrates may be improved via the combined exploitation of depressurization of the horizontal well and heat injection, and in order to reduce the exploitation costs of the natural gas hydrates, seawater is injected into the natural gas hydrate reservoirs along the first horizontal well 6 at a temperature of more than 60? C. to elevate the reservoir temperature; and meanwhile, a pressure at bottoms of the second horizontal well 8 and the third horizontal well 9 is reduced, so as to improve a decomposition rate of the natural gas hydrates in the reservoirs under the collaborative action of the two operations.
According to the above-mentioned technical solution, drilling time is shortened by rapid drilling along the horizontal well, the permeability of the reservoirs may be effectively improved by fracturing for fracture forming, the stability of the reservoirs may be improved by injecting foam cement slurry into the reservoirs, and the yield of the natural gas hydrates may be improved by the combined exploitation method of depressurization of the horizontal well and heat injection. The method has the advantages of reducing drilling costs, improving the permeability of the reservoirs, enhancing the stability of the reservoirs, improving the gas recovery efficiency and the like.

Example 2

(9) A high-efficiency yield-increasing exploitation method for natural gas hydrates is different from Example 1 in that in step (1), an interval among different horizontal wells is determined by the following method:

(10) Firstly, based on a Darcy's law, a pressure distribution in natural gas hydrate reservoirs may be as follows:

(11) $\begin{array}{cc}{P}_2=P_1+\frac{Q.Math.?.Math.L}{K.Math.A}& \left(1\right)\end{array}$

(12) where P.sub.1 is a bottom pore pressure of the horizontal wells, MPa; P.sub.2 is a pressure in the natural gas hydrate reservoirs, MPa; Q is a flow in pores of the natural gas hydrate reservoirs, m.sup.3/s; ? is a fluid viscosity in the reservoirs, mPa.Math.s; L is a seepage radius of a fluid in the reservoirs, m; K is an absolute permeability of the natural gas hydrate reservoirs, mD; A is a sectional area of seepage flow of the natural gas hydrate reservoirs, m.sup.2;

(13) then, in order to achieve a decomposition of the natural gas hydrates during an exploitation of the natural gas hydrates under reduced pressure, the pressure P.sub.2 in the reservoirs needs to conform to the following criteria:

(14) $\begin{array}{cc}{P}_2?P_e=10^6\exp \left(\underset{n=0}{\overset{5}{.Math.}}{a_n\left(T+?T_d\right)}^n\right)& \left(2\right)\end{array}$

(15) where

(16) $\begin{array}{cc}\{\begin{array}{c}{a}_0=-1.94138504464560?10^5\\ a_1=3.31018213397926?10^3\\ a_2=-2.25540264493806?10^1\\ a_3=7.67559117787059?10^{-2}\\ a_4=-1.30465829788791?10^{-4}\\ a_5=8.86065316687571?10^{-8}\end{array}& \left(3\right)\end{array}$

(17) where T is a reservoir temperature, K; ?T.sub.d is a temperature at which a decline in a hydrate equilibrium is caused by a thermodynamic hydrate inhibitor, K;

(18) ?T.sub.d may be obtained by the following formula through calculation:

(19) 0$\begin{array}{cc}?T_d=?T_{d,r}\frac{\ln \left(1-x\right)}{\ln \left(1-x_r\right)}& \left(4\right)\end{array}$

(20) where x is a molar fraction of the thermodynamic hydrate inhibitor in a water phase, which is dimensionless; x.sub.r is a reference molar fraction of the thermodynamic hydrate inhibitor in the water phase, which is dimensionless; ?T.sub.d,r is a temperature at which the decline in the hydrate equilibrium is caused under the molar fraction of the thermodynamic hydrate inhibitor as x.sub.r, K; and

(21) an interval (L.sub.1=L) between every two horizontal wells is solved according to formulas (1)-(4), based on which the layout of the three horizontal wells is supported theoretically.

Example 3

(22) A high-efficiency yield-increasing exploitation method for natural gas hydrates is different from Example 2 in that in step (2), during fracturing, a fracturing pressure is larger than a fracture forming pressure of the natural gas hydrate reservoirs.

(23) In step (2), a preparation process of foam cement slurry is as follows:

(24) a certain mass of cement is mixed with water to form cement slurry, and a certain mass of foaming agent (sodium dodecyl sulfate) and foam stabilizer (laurinol) are added to the cement slurry with stirring until fine and stable bubbles which are independent of each other are formed, and the foam cement slurry with low density and high permeability may be formed, with a density thereof being less than 1.0 g/cm.sup.3, which needs, however, to be determined according to actual design and requirements on site, that is, ?.sub.gfc in the formula (5). The construction method is readily available, without arranging any additional equipment, and the formed foam cement slurry has the advantages of stable properties, high compressive strength, low costs, and the like. What there will be to adopt in the construction method is thinner foam cement slurry. This may improve the liquidity of the foam cement slurry in the reservoirs, making it distributed all over the pores of the reservoirs better.

(25) In the preparation of the foam cement slurry, according to the properties of the required foam cement slurry and a mix proportioning principle thereof, the using amount of the cement is as follows:

(26) $\begin{array}{cc}{M}_{sn}=\frac{V_{\mathrm{gfc}}.Math.{?}_{\mathrm{gfc}}}{s_a}& \left(5\right)\end{array}$

(27) where M.sub.sn is the using amount of the cement, kg; ?.sub.gfc is the design dry density of the foam cement, kg/m.sup.3; V g fc is the volume of the dry foam cement, m.sup.3; S.sub.a is a mass coefficient, which is dimensionless, wherein it is 1.2 for ordinary Portland cement, and 1.4 for sulfate cement;

(28) water supply volume:
M.sub.w=?.Math.M.sub.sn(6)

(29) where M.sub.sn is the water supply volume; a is a basic ratio of water to materials, which is dimensionless;

(30) the using amount of the foaming agent:

(31) $\begin{array}{cc}{M}_p=\frac{V_{\mathrm{py}}.Math.{?}_{\mathrm{py}}}{b+1}& \left(7\right)\end{array}$

(32) where M.sub.p is the mass of the foaming agent in the foam cement slurry, kg; V.sub.py is the volume of a foam concentrate formed from the foaming agent, m.sup.3; ?.sub.py is the density of the foam concentrate formed from the foaming agent, kg/m.sup.3; b is the times of foaming of the foaming agent, which is dimensionless; and

(33) the using amount of the foam stabilizer is half of that of the foaming agent.

(34) The injection rate of the foam cement slurry is as follows:
V.sub.fc?2L.sub.1.Math.L.sub.2.Math.H.Math.S.sub.f(8)

(35) where V.sub.fc is a volume of the foam cement slurry to be injected into the natural gas hydrate reservoirs, m.sup.3; L.sub.2 is a length of the horizontal well section, m; H is a thickness of the natural gas hydrate reservoirs, m; and S.sub.f is a porosity of the natural gas hydrate reservoirs subjected to fracturing for fracture forming, which is dimensionless.

Example 4

(36) A high-efficiency yield-increasing exploitation method for natural gas hydrates is different from Example 3 in that in step (2), during the injection of foam cement slurry into the horizontal well section of the first horizontal well, when it is found that there is the foam cement slurry in the second horizontal well and the third horizontal well, the injection of the foam cement slurry may be stopped, and meanwhile, the foam cement slurry is removed rapidly from the second horizontal well and the third horizontal well, that is, clear water may be injected into drill stems in the second horizontal well and the third horizontal well, the foam cement slurry carried thereby returns upwards from an annular space between the drill stems and drivepipes to fulfill the objective of cleaning; and then, each of the horizontal wells is shut in for 48 h, until the foam cement slurry injected into the natural gas hydrate reservoirs is cured for forming, which may improve the stability of the reservoirs during the exploitation of the natural gas hydrates and reduce risks of the collapse and sand production of the natural gas hydrate reservoirs.

Example 5

(37) A high-efficiency yield-increasing exploitation method for natural gas hydrates is different from Example 4 in that, in step (3), the pressure distributed in the reservoirs is reduced by controlling a pressure decay amplitude at the bottoms of the second horizontal well and the third horizontal well, so that the natural gas hydrates in the reservoirs are decomposed into gases and water, and meanwhile, and the gases flow into the second horizontal well and the third horizontal well under a differential pressure for recovery

(38) In order to solve the problem of the secondary generation of the hydrates due to the rapid decomposition of the natural gas hydrates for a short time in the reservoirs, a relationship between absorption of heat from the decomposition of the hydrates and a heat transfer around the reservoirs is coordinated by controlling the bottom hole pressure of the second horizontal well and the third horizontal well with a multi-stage step-by-step depressurization strategy, that is, the pressure decay amplitude at the bottoms of the wells is reduced slowly after the bottom hole pressure is reduced to a hydrate phase equilibrium condition in decomposition, ranging from 0.1 to 0.2 MPa, and with every 0.5 MPa of the bottom hole pressure declining, a bottom hole pressure value is maintained until a gas recovery rate declines significantly (i.e., declining to 1000 cubic meters/day below) before the depressurization of the next step, thereby fulfilling the objective of reducing or avoiding the secondary generation risk of the hydrates in the reservoirs.

(39) A method for reducing the bottom pore pressure is as follows:

(40) Before the exploitation of the hydrates, the wellhole is filled with water, at which the bottom pore pressure is equal to a gravity pressure of water; and after exploitation starts, the water will be pumped to a platform from the wellhole, and with a decrease in the water volume in the wellhole, the bottom pore pressure will decline gradually, thereby reducing the pressure in the reservoirs.

(41) The above descriptions are only preferred implementations of the prevent invention, and it should be noted that a person of ordinary skill in the art can further make several improvements and modifications without departing from the principle of the present invention, and those improvements and modifications should be included in the protection scope of the present disclosure.