Method and device for determining elasticity of cement stone utilized in well cementing of oil-gas well

Abstract

The invention provides a method and device for measuring the elasticity of hardened cement for cementing of oil-gas wells. The measurement method comprises: determining the loading and unloading rates of the hardened cement; determining the maximum loading on the hardened cement; determining the experimental temperature and the experimental pressure of the hardened cement; establishing a stress-strain curve for the hardened cement; and describing the elasticity of the hardened cement with the degree of strain recovery of the hardened cement in different cycles, and describing the mechanic integrity of the hardened cement with the degree of damage to the hardened cement in different cycles. The invention further provides a device for measuring the elasticity of hardened cement for cementing of oil-gas wells. The measurement method and device of the present invention provide a universal comparing platform for research on hardened cement modification as well as examination of domestic and foreign special cement slurry systems, which is of great significance in evaluation of well hole integrity and well life.

Claims

1. A method for measuring the elasticity and mechanical integrity of hardened cement for cementing of oil-gas wells, the method comprising the steps of: i) determining the loading and unloading rates of the hardened cement, according to the force applied on the hardened cement and the duration required for the loading and unloading of the force, in various engineering operations; ii) establishing a normal stress-strain curve, and determining the maximum loading on the hardened cement by comparing the data of the force applied on the hardened cement in the various engineering operations to the normal stress-strain curve; iii) determining the experimental temperature and the experimental pressure, and establishing a stress-strain curve for the hardened cement by conducting a multi-cycle tri-axial stress test on the hardened cement at the experimental temperature and the experimental pressure based on the determined loading rate, unloading rate and maximum loading of the hardened cement; and obtaining a quantitative evaluation of the elasticity of the hardened cement with the degree of strain recovery of the hardened cement in different cycles; and iv) obtaining a qualitative evaluation of the mechanic integrity of the hardened cement by conducting a multi-cycle mechanic test by a testing method using alternating loadings.

2. The method according to claim 1, characterized by further comprising a step of preparing an experimental sample of hardened cement, prior to the measurement of the elasticity of the hardened cement for cementing of oil-gas wells, the preparing comprising: preparing an experimental cement slurry, setting and hardening the cement slurry into hardened cement by curing the cement slurry under a simulated temperature and pressure condition for hardened cement for a designated period of time according to the downhole environment surrounding the hardened cement, and processing the hardened cement to a standard core size to obtain the sample of hardened cement.

3. The method according to claim 1, characterized in that the normal stress-strain curve is established through a tri-axial stress test.

4. The method according to claim 1, characterized in that the determining the maximum loading on the hardened cement comprises the step of: determining the average value of the maximum strains of the hardened cement, and determining the maximum loading as the stress value corresponding to the maximum strain according to the normal stress-strain curve.

5. The method according to claim 1, characterized in that the experimental temperature and the experimental pressure are determined in accordance with the downhole depth of the hardened cement.

6. The method for according to claim 1, characterized in that the degree of strain recovery of the hardened cement in different cycles is determined according to the following equation:
(maximum strain upon loadingminimum strain upon unloading)/maximum strain upon loading.

7. The method according to claim 1, characterized in that for the qualitative evaluation of the mechanic integrity of the hardened cement, if the hardened cement shows microcracks or breaks, the hardened cement cannot withstand the mechanical impacts from various subsequent engineering operations, indicating lack of mechanic integrity; or if the hardened cement does not show microcracks or break, the hardened cement can withstand the mechanical impacts from various subsequent engineering operations, indicating mechanic integrity.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a structural schematic representation of a device for measuring the elasticity of hardened cement for cementing of oil-gas wells according to an example.

(2) FIG. 2 is a graph of the stress-strain curve of pure cement according to an example.

(3) FIG. 3 is a graph of the stress-strain curve of pure cement according to an example under alternating loadings.

(4) FIG. 4 is a graph of the stress-strain curve of #1 the Tough Cement according to an example.

(5) FIG. 5 is a graph of s the tress-strain curve of #1 the Tough Cement according to an example under alternating loadings.

(6) FIG. 6 is a graph of the stress-strain curve of #2 the Micro Expansive Cement according to an example.

(7) FIG. 7 is a graph of the stress-strain curve of #2 the Micro Expansive Cement according to an example under alternating loadings.

(8) FIG. 8 is a graph of the stress-strain curve of #3 the Elastic Cement according to an example.

(9) FIG. 9 is a graph of the stress-strain curve of #3 the Elastic Cement according to an example under alternating loadings.

DETAILED DESCRIPTION

(10) For a better understanding of the technical features, objectives and beneficial effects of the present invention, description is provided in detail for the technical solutions of the invention, but is not intended to be construed as limiting the implementable scope of the present invention.

(11) In the following Examples, an on-site sample taken from the cement slurry for well cementing in the 7-inch liner of the X gas well in the Chuanyu gas field (#1 the Tough Cement system, provided by the downhole operation division of the CNPC Chuanqing Drilling Engineering Company Ltd., density: 2.30 g/cm.sup.3) and pure cement (Jiahua cement, grade water/cement ratio: 0.44, density: 1.90 g/cm.sup.3) were used; curing conditions: at a temperature of 119 C. and pressure of 20.7 MPa.

Example 1

(12) This example provides a device for measuring the elasticity of hardened cement for cementing of oil-gas wells. The device has a structure as shown in FIG. 1, and specifically comprises: a module for determining loading and unloading rates, which determines the loading and unloading rates of the hardened cement according to the force applied on the hardened cement and on the duration required for the loading and unloading of the force in various engineering operations; a module for determining a maximum loading, which determines the maximum loading on the hardened cement by comparing the data of the force applied on the hardened cement in the various engineering operations to a normal stress-strain curve; a module for establishing a stress-strain curve, which establishes the stress-strain curve of the hardened cement by conducting a multi-cycle tri-axial stress test on the hardened cement at the experimental temperature and the experimental pressure based on the determined loading rate, unloading rate and maximum loading of the hardened cement; and an analytical module, which quantitatively describes the elasticity of the hardened cement with the degree of strain recovery of the hardened cement in different cycles, and qualitatively describes the mechanic integrity of the hardened cement with the degree of damage to the hardened cement in different cycles.

(13) This example provides a method for measuring the elasticity of hardened cement for cementing of oil-gas wells, specifically comprising the steps of: taking cement bulk samples and on-site liquid from the well-cementing site; preparing and curing the cement sampled on-site (slurry system of #1 the Tough Cement, density: 2.30 g/cm.sup.3) and pure cement (water/cement ratio: 0.44, density: 1.90 g/cm.sup.3) according to the API standard; upon completion of curing at a high temperature and high pressure (curing temperature: 119 C., curing pressure: 20.7 MPa, curing period: 7 days), taking the core or directly curing the cement with a standard rock core mold to prepare a hardened cement sample having a standard rock core size (25.4 mm50.8 mm); determining the loading and unloading rates: with the Software System for Analysis of Formation Cement Sheath Mechanics (Chinese Software Writing Registration No. 0910640) developed by the Southwest Oil&Gasfield Company, the loading and unloading rates were calculated as 1.6 kN/min and 3.2 kN/min, respectively, for the well cementing with a 177.8 mm casing at a well depth of 5000 m, with a drilling fluid at a density of 2.20 g/cm.sup.3, and at a testing pressure of 30 MPa; establishing a normal stress-strain curve: performing a tri-axial stress test on the pure cement and #1 the Tough Cement at a loading rate of 1.6 kN/min until the hardened cement was damaged, so as to obtain a normal stress-strain curve, as shown in FIGS. 2 and 4; determining the maximum loading under alternating loadings: with the Software System for Analysis of Formation Cement Sheath Mechanics (Chinese Software Writing Registration No. 0910640), the maximum strain of the cement sheath was calculated as 0.1811% for the well cementing with a 177.8 mm casing at a well depth of 5000 m, with a drilling fluid at a density of 2.20 g/cm.sup.3, and at a testing pressure of 30 MPa; and the corresponding stress values found in the graph of the stress-strain curve of the pure cement shown in FIG. 2 and in the graph of the stress-strain curve of Tough Cement 1 shown in FIG. 4 were 8.6 MPa and 8.7 MPa, respectively, which were used as the maximum loading of the alternating loadings; and

(14) establishing a stress-strain curve by conducting a tri-axial stress test under 6 cycles of alternating loadings on the pure cement and #1 the Tough Cement, with the loading rate of 1.6 kN/min and the unloading rate of 3.2 kN/min and the maximum loadings of 8.6 MPa and 8.7 MPa, respectively, as shown in FIGS. 3 and 5.

(15) The degree of strain recovery of the hardened cement in each cycle was calculated according to the following equation: (maximum strain upon loadingminimum strain upon unloading)/maximum strain upon loading; which was used to compare and evaluate the elasticity/flexibility/toughness of the two hardened cements. Moreover, under the alternating loadings, the degree of damage to the hardened cements in different cycles (presence or absence of cracks or breaking on the hardened cement) was observed and used to compare and evaluate the long-term mechanic integrity of the two hardened cements.

(16) FIGS. 3 and 5 are graphs of stress-strain curves of the pure cement and #1 the Tough Cement system from the tests under alternating loadings in consideration of the pressure-testing engineering operations (well cementing with a 177.8 mm casing at a well depth of 5000 m, with a drilling fluid at a density of 2.20 g/cm.sup.3, and at a testing pressure of 30 MPa). As can be seen from the graphs, in each cycle both hardened cements underwent a process in which the strain decreases at various degrees with the decrease in the stress; in other words, both hardened cements showed certain elasticity (or flexibility or toughness) under alternating loadings. For a comparative analysis, the degree of strain recovery of the two hardened cements in each cycle was used to quantitatively describe the elasticity (or flexibility or toughness) thereof, and the degree of damage to the hardened cements occurring in each cycle was used to qualitatively describe the long-term mechanic integrity of the hardened cements.

(17) 1. Degree of Strain Recovery of the Hardened Cement

(18) From the curves in FIGS. 3 and 5, the degrees of strain recovery of the pure cement in the 6 cycles of alternating loadings were 0.072, 0.067, 0.061, 0.056, 0.051, and 0.050, respectively; the degrees of strain recovery of #1 the Tough Cement in 5 cycles of alternating loadings were 0.0461, 0.0561, 0.0593, 0.0572, and 0.0609, respectively. #1 the Tough Cement was damaged in the 6.sup.th cycle of alternating loadings.

(19) (1) When the degrees of strain recovery of the hardened cements in the 1.sup.st cycle are compared, #1 the Tough Cement had a lower degree of strain recovery than that of the pure cement, which did not mean that the elasticity or flexibility or toughness of the pure cement was better than that of #1 the Tough Cement. Upon analysis, it is believed that the pure cement was a hardened cement having a regular density of 1.90 g/cm.sup.3 formulated with a grade G cement for oil wells at a water/cement ratio of 0.44 (its engineering properties, including stability, thickening time and the like, do not meet relevant regulations and requirements) and having poor settling stability, and therefore the hardened cement formed was relatively dense, and directly entered the elasticity/flexibility/toughness deformation phase during loading. In contrast, for #1 the Tough Cement, because various external additives for enhancing toughness, elasticity, or flexibility were added in the system of #1 the Tough Cement in consideration of the overall engineering performance, and the stable system resulted in a certain pore space within the formed crystalline structure, there was a compressed phase during the initial loading and unloading, which phase did not reflect the mechanic deformation capacity in terms of the elasticity/flexibility/toughness of the system. As seen at later stages, starting from the 4.sup.th cycle, the degree of strain recovery of #1 the Tough Cement became higher than that of the pure cement.

(20) (2) Based on the results from alternating loadings, the result of comparison of the elasticity of the two hardened cements is: hardened #1 the Tough Cement>hardened pure cement.

(21) 2. Degree of Damage to the Hardened Cement

(22) As observed during the test of the two hardened cements under alternating loadings, #1 the Tough Cement became damaged in the 6.sup.th cycle of alternating loadings, showing the result of comparison of the long-term mechanic integrity of the two hardened cements: hardened pure cement>hardened #1 the Tough Cement.

Example 2

(23) This example provides a method for measuring the elasticity of hardened cement for cementing of oil-gas wells, specifically comprising the steps of: taking cement bulk samples and on-site liquid twice from the well-cementing site of the injection-production well at the gas storage (a system of #3 the Elastic Cement, provided by Schlumberger Ltd.; and a system of #2 the Micro Expansive Cement, provided by the CNPC Engineering Technology R&D Company Ltd.); preparing and curing the cement sampled on-site (#3 the Elastic Cement, density: 1.75 g/cm.sup.3, and #2 the Micro Expansive Cement, density: 1.75 g/cm.sup.3) according to the API standard; upon completion of curing at a high temperature and high pressure (curing temperature: 58 C., curing pressure: 20.7 MPa, curing period: 7 days), taking the core or directly curing the cement with a standard rock core mold to prepare hardened cement samples with a standard rock core size (25.4 mm50.8 mm); determining the loading and unloading rates: with the Software System for Analysis of Formation Cement Sheath Mechanics (Chinese Software Writing Registration No. 0910640) developed by the Southwest Oil&Gas field Company, the loading and unloading rates were calculated as 0.5 kN/min and 2.0 kN/min respectively, for the well cementing with a 177.8 mm casing at a well depth of 3000 m, with gas production of 6010.sup.4 m.sup.3/d and gas injection of 9010.sup.4 m.sup.3/d; establishing a normal stress-strain curve: performing a tri-axial stress test on #2 the Micro Expansive Cement and #3 the Elastic Cement at a loading rate of 0.5 kN/min until the hardened cement was damaged, so as to obtain a normal stress-strain curve, as shown in FIGS. 6 and 8; determining the maximum loading under alternating loadings: with the Software System for Analysis of Formation Cement Sheath Mechanics (Chinese Software Writing Registration No. 0910640), the maximum strain of the cement sheath was calculated as 0.1624% for the well cementing with 177.8 mm casing at a well depth of 3000 m, with gas production of 6010.sup.4 m.sup.3/d and gas injection of 9010.sup.4 m.sup.3/d, and the corresponding stress values found in the graph of the stress-strain curve of #2 the Micro Expansive Cement shown in FIG. 5 and in the graph of the stress-strain curve of #3 the Elastic Cement shown in FIG. 7 were 8.5 MPa and 7.8 MPa, respectively, which were used as the maximum loading of the alternating loadings; and establishing a stress-strain curve by conducting a tri-axial stress test under 6 cycles of alternating loadings on #2 the Micro Expansive Cement and #3 the Elastic Cement, with the loading rate of 0.5 kN/min and the unloading rate of 2.0 kN/min and the maximum loadings of 8.5 MPa and 7.8 MPa, respectively, as shown in FIGS. 7 and 9.

(24) The degree of strain recovery of the hardened cement in each cycle was calculated according to the following equation: (maximum strain upon loadingminimum strain upon unloading)/maximum strain upon loading; which was used to compare and evaluate the elasticity/flexibility/toughness of the two hardened cements. Moreover, under the alternating loadings, the degree of damage to the hardened cements in different cycles (presence or absence of cracks or breaking on the hardened cement) was observed and used to compared and evaluate the long-term mechanic integrity of the two hardened cements.

(25) 1. Degree of Strain Recovery of the Hardened Cement

(26) From the curves in FIGS. 7 and 9, the degrees of strain recovery of #2 the Micro Expansive Cement in the 6 cycles of alternating loadings were 0.0681, 0.768, and 0.074, respectively, with cracks appearing in the 4.sup.th cycle of alternating loadings; the degrees of strain recovery of #3 the Elastic Cement in 6 cycles of alternating loadings were 0.0842, 0.082, 0.082, 0.067, 0.0784, and 0.0762 respectively; #3 the Elastic Cement maintained its integrity and had a higher degree of strain recovery than that of #2 the Micro Expansive Cement during first three cycles of alternating loadings.

(27) Based on the results from alternating loadings, the result of comparison of the elasticity of the two hardened cements is: #3 the Elastic Cement>#2 the Micro Expansive Cement.

(28) 2. Degree of Damage to the Hardened Cement

(29) As observed during the test of the two hardened cements under alternating loadings, in #2 the Micro Expansive Cement became damaged in the 3.sup.th cycle of alternating loadings, while #3 the Elastic Cement maintained its integrity after the 6 cycles of alternating loadings, showing the result of comparison of the long-term mechanic integrity of the two hardened cements: #3 the Elastic Cement>#2 the Micro Expansive Cement.

(30) The system, device, module, or unit as illustrated in the above Examples can be implemented by a computer chip or entity, or implemented by a product with a certain function.

(31) For the ease of description, various modules defined by function are separately described for the above device. However, the functions of the various modules can certainly be implemented in one or more than one piece of software and/or hardware when the present invention is implemented.

(32) Based on the description of the above embodiments, those skilled in the art may clearly understand that the present invention can be implemented by means of software in combination with a general hardware platform. Based on such understanding, it is possible to embody the essence of the technical solutions of the present application or the part making a contribution over the prior art in the form of a software product, and in a typical configuration, a computing device includes one or more processors (CPU), an input/output interface, a network interface and a memory.

(33) Each of the examples in the specification is described in a progressive manner, with the same or similar parts in each of the examples referential to each other, and the part that distinguishes an example from other example(s) is described by emphasis. In particular, for an example of system, description is relatively concise because it is similar to an example of method, and reference can be made to the counterpart described in the example of method.

(34) The present application can be described in the general context of computer executable instructions executed by a computer, such as a program module. Generally, a program module includes a routine, a program, an object, a component, a data structure or the like that executes a particular task or embody a particular type of abstract data. The present application may also be practiced in a distributed computing environment where a remote processing device connected by communication networks executes the task. In a distributed computing environment, a program module may be located in the local and remote computer storage media including a storage device.

(35) As demonstrated in the above examples, the method and device for measuring the elasticity of hardened cement for cementing of oil-gas wells according to the present invention provide a universal comparing platform for research on hardened cement modification as well as examination of domestic and international special cement slurry systems, and provides a strong technical support for real representation of the nature of mechanics of the downhole cement sheath, which is of great significance in evaluation of the integrity of the well hole and the lifetime of a well.