RISK ASSESSMENT-BASED DESIGN METHOD FOR DEEP COMPLEX FORMATION WELLBORE STRUCTURE

Abstract

A risk assessment-based design method for a deep complex formation wellbore structure includes: (1) preliminarily determining casing layers and setting depths; (2) calculating to obtain the risk coefficients of each layer of casing; (3) analyzing and coordinating, according to the principle that a shallow casing shares more risks and a deep casing shares less risks, the risks of each layer of casing: determining whether the risk coefficients of each layer of casing are greater than a safety threshold value K; checking the setting depth: if the safety coefficient of an ith-layer casing satisfies R.sub.Ni>K, selecting a casing layer with the minimum safety coefficient from upper casing layers, and deepening the setting depth h of the casing layer; and (4) repeating the steps (2) to (3) until the casing risk coefficients of each layer of casing are less than the safety threshold value K.

Claims

1. A risk assessment-based drilling method with a deep complex formation wellbore structure, wherein the deep complex formation wellbore structure is controlled by a device that comprises a computer-readable device and an instruction, and the device executes a processor for performing the following steps: (i) generating, by the processor, casing layers and a setting depth; (ii) creating a probabilistic distribution of formation pressure and prediction accuracy, by the processor, according to a prediction error of formation pressure and a well depth wherein the prediction error of formation pressure is correlated to the well depth;  wherein a probabilistic distribution of the prediction error of formation pressure is created, by the processor, based on a standard deviation and the prediction accuracy;  creating, by the processor, a cumulative probability corresponding to the formation pressure by integrating the probabilistic distribution of the prediction error of formation pressure wherein the cumulative probability is greater than the prediction error of formation pressure; creating, by the processor, a probabilistic distribution of wellbore structure design coefficient based on a threshold value of a safety coefficient and a standard deviation of the threshold value of the safety coefficient;  generating, by the processor, a distribution interval of the threshold value of the safety coefficient with a credibility and the cumulative probability; wherein the probabilistic distribution of wellbore structure design coefficient is greater than the cumulative probability, any specific value of the cumulative probability is within the range of the probabilistic distribution of wellbore structure design coefficient;  creating, by the processor, a downhole engineering risk at a specific well depth according to a pressure balance relationship which is created by the processor with a kick risk; the kick risk is resulted from a risk of lost circulation, an equivalent density of drilling fluid, a formation pore pressure, a minimum limit of the formation pore pressure, and the depth of a last casing shoe; wherein a wellbore pressure is less than the sum of the formation pore pressure and the probabilistic distribution of wellbore structure design coefficient; creating, by processor, a risk coefficient by integrating the downhole engineering risk, wherein a specific downhole engineering risk is within a specific well structure scheme; (iii) selecting, by the processor, a specific risk coefficient of each layer of casing; wherein the specific risk coefficient of each layer of casing is greater than the threshold value of the safety coefficient; selecting, by the processor, a specific setting depth wherein the safety coefficient is greater than the threshold value of the safety coefficient, selecting a casing layer with a minimum safety coefficient from the casing layers, and deepening the setting depth of the casing layer; (iv) repeating (ii) to (iii) until the risk coefficient of each layer of casing are less than the threshold value of the safety coefficient; and (v) positioning the deep complex formation wellbore structure for drilling based on the risk coefficient which provides data for overall planning and quantitative optimization of casing running depth at each level before drilling.

2. The risk assessment-based drilling method according to claim 1, wherein the method in the step (i) further comprises: (a) measuring a geological setting position; (b) creating, by the processor, a safety pressure window based on prediction results of formation pore pressure, formation fracture pressure and formation collapse pressure before drilling and a pressure balance relationship of an open hole section; and (c) generating, by the processor, the casing layers and the setting depth based on the geological setting position, the safety pressure window and a regional wellbore structure design coefficient.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0062] FIG. 1 is a specific design contrast diagram for a wellbore structure in an embodiment of the present invention.

[0063] FIG. 2 is a flowchart of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0064] A specific implementation mode is introduced by taking a well A as an example. The design well depth is 6500 m; the kick tolerance S.sub.k=0.05 g/cm.sup.3, the formation fracture pressure safety factor S.sub.t=0.04 g/cm.sup.3, the additional drilling fluid density Δρ=0.05 g/cm.sup.3 and the suction pressure coefficient S.sub.b=0.04 g/cm.sup.3. A formation pressure profile is as shown in FIG. 1.

[0065] A wellbore structure scheme of the well is preliminarily determined by adopting a top to bottom method according to (1) to (3) of the present invention.

[0066] In (2), the error cumulative probability formulas of the formation pore pressure and the formation fracture pressure are respectively obtained as follows by selecting the standard deviation of the formation pressure prediction error σ.sub.P.sub.i=0.6:

[0067] formation pore pressure:

[00007]$P\left(P_{Pi}\right)={\int }_{-\infty }^{P_{\mathrm{pi}}}\frac{1}{\sigma \sqrt{2\pi }}{e}^{-\frac{{\left(P_{\mathrm{pi}}-\frac{P_{\mathrm{pi}1}+P_{\mathrm{pi}0}}{2}\right)}^2}{2{\sigma }^2}}{\mathrm{dP}}_{\mathrm{pi}}={\int }_{-\infty }^{P_{\mathrm{pi}}}\frac{5}{3\sqrt{2\pi }}{e}^{-\frac{25{\left(P_{\mathrm{pi}}-\frac{P_{\mathrm{pi}1}+P_{\mathrm{pi}0}}{2}\right)}^2}{18}}{\mathrm{dP}}_{\mathrm{pi}}$

[0068] formation fracture pressure:

[00008]$P\left(P_{fi}\right)={\int }_{-\infty }^{P_{\mathrm{fi}}}\frac{1}{\sigma \sqrt{2\pi }}{e}^{-\frac{{\left(P_{\mathrm{fi}}-\frac{P_{\mathrm{fi}1}+P_{\mathrm{fi}0}}{2}\right)}^2}{2{\sigma }^2}}{\mathrm{dP}}_{\mathrm{fi}}={\int }_{-\infty }^{P_{\mathrm{fi}}}\frac{5}{3\sqrt{2\pi }}{e}^{-\frac{25{\left(P_{\mathrm{fi}}-\frac{P_{\mathrm{fi}1}+P_{\mathrm{fi}0}}{2}\right)}^2}{18}}{\mathrm{dP}}_{\mathrm{fi}}$

[0069] According to the drilling experience of an adjoining well in the region, kick and lost circulation easily occur in the downhole with the depth interval of 4000 to 5000 m, so that the wellbore structure design coefficient σ.sub.P.sub.i=0.7 is selected for the depth greater than 4000 m, and σ.sub.P.sub.i=0.5 is selected for other depths. The credibility J=90% is set to obtain the distribution intervals and the cumulative probability calculation formulas of each of the coefficients as follows:

[0070] Kick tolerance: the distribution interval is

[00009]$\hspace{1em}\{\begin{array}{cc}\left[0.04,0.06\right]& H<4000m\\ \left[0.036,0.064\right]& H\ge 40000m\end{array}$

[0071] the cumulative probability formula is

[00010]$P\left(f_i\left(S_K\right)\right)=\{\begin{array}{cc}{\int }_{-\infty }^{f_i\left(S_K\right)}\sqrt{\frac{2}{\pi }}{e}^{-2{\left(f_i\left(S_K\right)+0.05\right)}^2}d\left(f_i\left(S_K\right)\right)& H<4000m\\ {\int }_{-\infty }^{f_i\left(S_K\right)}\sqrt{\frac{50}{49\pi }}{e}^{-\frac{2{\left(f_i\left(S_K\right)+0.05\right)}^2}{49}}d\left(f_i\left(S_K\right)\right)& H\ge 40000m\end{array}$

[0072] Formation fracture pressure safety factor: the distribution interval is

[00011]$\hspace{1em}\{\begin{array}{cc}\left[0.032,0.048\right]& H<4000m\\ \left[0.03,0.05\right]& H\ge 40000m\end{array}$

[0073] the cumulative probability formula is

[00012]$P\left(f_i\left(S_f\right)\right)=\{\begin{array}{cc}{\int }_{-\infty }^{f_i\left(S_f\right)}\sqrt{\frac{2}{\pi }}{e}^{-2{\left(f_i\left(S_f\right)+0.04\right)}^2}d\left(f_i\left(S_f\right)\right)& H<4000m\\ {\int }_{-\infty }^{f_i\left(S_f\right)}\sqrt{\frac{50}{49\pi }}{e}^{-\frac{2{\left(f_i\left(S_f\right)+0.04\right)}^2}{49}}d\left(f_i\left(S_f\right)\right)& H\ge 40000m\end{array}$

[0074] Additional drilling fluid density: the distribution interval is

[00013]$\hspace{1em}\{\begin{array}{cc}\left[0.04,0.06\right]& H<4000m\\ \left[0.036,0.064\right]& H\ge 40000m\end{array}$

[0075] the cumulative probability formula is

[00014]$P\left(f_i\left(S_K\right)\right)=\{\begin{array}{cc}{\int }_{-\infty }^{f_i\left(S_K\right)}\sqrt{\frac{2}{\pi }}{e}^{-2{\left(f_i\left(S_K\right)+0.05\right)}^2}d\left(f_i\left(S_K\right)\right)& H<4000m\\ {\int }_{-\infty }^{f_i\left(S_K\right)}\sqrt{\frac{50}{49\pi }}{e}^{-\frac{2{\left(f_i\left(S_K\right)+0.05\right)}^2}{49}}d\left(f_i\left(S_K\right)\right)& H\ge 40000m\end{array}$

[0076] Suction pressure coefficient: the distribution interval is

[00015]$\hspace{1em}\{\begin{array}{cc}\left[0.032,0.048\right]& H<4000m\\ \left[0.03,0.05\right]& H\ge 4000m\end{array}$

[0077] The cumulative probability formula is

[00016]$P\left(f_i\left(S_f\right)\right)=\{\begin{array}{cc}{\int }_{-\infty }^{f_i\left(S_f\right)}\sqrt{\frac{2}{\pi }}{e}^{-2{\left(f_i\left(S_f\right)+0.04\right)}^2}d\left(f_i\left(S_f\right)\right)& H<4000m\\ {\int }_{-\infty }^{f_i\left(S_f\right)}\sqrt{\frac{50}{49\pi }}{e}^{-\frac{2{\left(f_i\left(S_f\right)+0.04\right)}^2}{49}}d\left(f_i\left(S_f\right)\right)& H\ge 40000m\end{array}$

[0078] According to (2) to (3) in the present invention, in the embodiment, there are five layers of casings in total, and the downhole engineering risks of each layer of casing at different well depths are respectively calculated:

[0079] a first-layer casing: the kick risk R.sub.JY=0; the lost circulation risk R.sub.JL=0;

[0080] a second-layer casing: the kick risk

[00017]$R_{JY}=\{\begin{array}{cc}0& H<2000m\\ 9\times 10^6{x}^2-0.035x+35.279& H\ge 2000m\end{array};$

[0081] the lost circulation risk R.sub.JL=0;

[0082] a third-layer casing: the kick risk

[00018]$R_{JY}=\{\begin{array}{cc}0& H<3500m\\ 2\times 10^6{x}^2-0.0127x-23.657& H\ge 3500m\end{array};$

[0083] the lost circulation risk

[00019]$R_{\mathrm{JL}}=\{\begin{array}{cc}2\times 10^6{x}^2-0.0091x+10.052& H<2150m\\ 0& H\ge 2150m\end{array};$

[0084] a fourth-layer casing: the kick risk

[00020]$R_{JY}=\{\begin{array}{cc}0& H<5150m\\ 2\times 10^6{x}^2-0.0217x+56.027& H\ge 5150m\end{array};$

[0085] the lost circulation risk

[00021]$R_{\mathrm{JL}}=\{\begin{array}{cc}-9\times {10}^{-7}{x}^2+0.0067x-11.819& H<3700m\\ 0& H\ge 3700m\end{array};$

[0086] a fifth-layer casing: the kick risk

[00022]$R_{JY}=\{\begin{array}{cc}0& H<6380m\\ 2\times 10^6{x}^2-0.0255x+81.718& H\ge 6380m\end{array};$

[0087] the lost circulation risk

[00023]$R_{JL}=\{\begin{array}{cc}-5\times 10^7{x}^2+0.0055x-15.312& H<5400m\\ 0& H\ge 5440m\end{array}.$

[0088] According to (2) to (4) in the present invention, the overall risk coefficient of each layer of casing is obtained:

R.sub.1=0; R.sub.2=∫.sub.300.sup.2100(R.sub.JY(H)+R.sub.JL(H))dH=0.532;

R.sub.3=∫.sub.2100.sup.3600(R.sub.JY(H)+R.sub.JL(H))dH=0.483; R.sub.4=∫.sub.3600.sup.5100(R.sub.JY(H)+R.sub.JL(H))dH=0.447;

R.sub.2=∫.sub.5100.sup.6500(R.sub.JY(H)+R.sub.JL(H))dH=0.408.

[0089] According to (3) to (4) in the present invention:

[0090] i): a safety threshold value K=0.5 is set according to actual conditions, wherein the overall risk coefficient of the second-layer casing is greater than the value;

[0091] ii): the setting depth of the first-layer casing is increased by 50 m;

[0092] iii): if the safety coefficient of the ith-layer casing satisfies R.sub.Ni>K, the casing layer with the minimum safety coefficient is selected from upper casing layers, and the setting depth h of the casing layer is deepened; and

[0093] iv): until the risk coefficient of each layer of casing is less than the safety threshold value K, the safety threshold value K of ranges is from 0.4 to 0.5, the preferred value K is 0.5;

[0094] v): Step iv) is used to determine the casing running depth for the comprehensive risk of the whole wellbore structure is reduced to the maximum extent, and a guarantee for safe and efficient drilling is provided.

[0095] In order to reflect the technical advantages of the present invention, a comparison is made between embodiments of the present invention and comparative examples, where the comparative example described in Table 1 refers to a comparative technical scheme formed according to (1) to (2) of the present invention.

TABLE-US-00001 TABLE 1 Comparative Example The Present Embodiment Drilling Drilling Casing Fluid Casing Fluid Casing Setting Density Risk Setting Density Risk Layer Depth (g/cm.sup.3) Factor Depth (g/cm.sup.3) Factor 1 300 m 1.17 0 350 m 1.18 0.035 2 2100 m 1.35 0.532 2125 m 1.37 0.0486 3 3600 m 1.68 0.483 3635 m 1.72 0.043 4 5100 m 2.07 0.447 5110 m 2.23 0.0421 5 6500 m 2.63 0.408 6500 m 2.62 0.0413

[0096] In combination with Table 1 and FIG. 1, it can be seen that, after the processing and design of the method disclosed by the present invention, the risks of the five layers of casings are all less than the safety threshold value K=0.5, the casing setting depth of the shallow formation is deeper, and the depth of the open hole section of the deep formation (the setting depths of the fourth and fifth layers of casings) is reduced, which facilitates the reduction of downhole risk of the deep formation drilling, transfers the risk of a deep casing layer to a shallow casing layer, and reduces the overall risk.