Stress Sensitivity And Pressure Drop Propagation Law Of Coal Reservoir During Drainage Process Of CBM Vertical Well

At present, the CBM production of a single well is generally low, unstable and of fast attenuation in our country. On the one hand, it is due to the poor physical conditions to our country’s coal reservoir, which is mainly reflected in the complexity of coal structure, low reservoir permeability and its strong stress sensitivity, and poor mechanical properties. On the other hand, it is because our draining technologies of CBM well are imperfect, such as the unreasonable production system. The CBM production effect is comprehensively decided by the two aspects. So it is of important theoretical and practical significances for obtaining a reasonable production system to strengthen the study on permeability dynamic evolution characteristics of coal reservoir and the propagation law of reservoir pressure drop in the process of drainage.In this paper, the coal reservoir of east Qinnan-Xiadian block in southeastern Qinshui basin was taken as the main research object. Through coal reservoir physical property testing, theoretical analyses and numerical simulation, research work of four aspects were carried out, such as detecting the pore-fracture structure characteristics of high-rank coal reservoir, analyzing the stress sensitivity of coal reservoir permeability through experimental study, building models of porosity and permeability evolution and fluid migration in coal reservoir, and simulating the propagation characteristics of pressure drop in the process of drainage, with the comprehensive application of the CBM development geology, surface physics and oil/gas seepage theories. Some achievements and cognitions obtained are as follows.(1) The exploration experiment results about high-rank coal pore-fracture structure have shown that, transitional pores(10-100 nm) are the main contributors for pore volume and specific surface area of adsorption-diffusion pores( < 0.1 μm) of undeformed and granulated-mylonitized coals. Micropores(1.7-10 nm) are commonly not developed. Adsorption-disffusion pores at the level of nanometer of granulated-mylonitized coal are more developed than those of undeformed coal, which is the main reason why deformed coal has better gas diffusion and adsorption abilities.Coal seepage pores(> 0.1 μm) could be obviously divided into two pore size ranges, such as large seepage pores and middle-small seepage pores. Large seepage pores of undeformed coal are more developed than granulated-mylonitized coals’, but the contrary is the case for middle-small seepage pores. Through fractal analysis of seepage pores and SEM observation, we can see that the lower limit of aperture size for large seepage pores of two kinds of coal actually lies between 5-6 microns. Tectonic stress is the primary cause for pore-fracture structure difference between the two coals. It could be obtained that coal reservoir primary seepage pore-fracture system generally consists of macro cleat-fractures and large seepage pores. Its development degree directly controls the permeability of coal reservoir. The primary seepage pore-fracture system of undeformed coal is generally developed, which makes it has good permeability. The actual lower limit of aperture size for large seepage pores of coal reservoir could be obtained by fractal analysis of seepage pores.(2) Confining pressure sensitivity, back pressure sensitivity and depressurization strength simulation experiment research for coal were carried out. The dynamic change rules of coal reservoir porosity and permeability were thoroughly discussed in the process of CBM well production. The confining pressure sensitivity experiment results show that there is a typical negative exponential function relationship between coal permeability and confining pressure. In the early stage of confining pressure increasing, coal stress sensitivity is relatively strong, and then fades out late, due to the deformation characteristics of coal pore-fracture structure under the action of effective stress. It could be speculated that raw deformed coal also has strong confining pressure sensitivity. Based on the characteristics of CBM vertical well drainage, back pressure sensitivity experiment method for coal sample was designed. Namely, change back pressure of coal sample to simulate the actual variation of CBM well bottom hole pressure and test the permeabilities of coal sample under different back pressures. With the loss of back pressure, coal permeability also decreases in exponential law.Because effective stress of coal skeleton changes in different ways caused by external stress and internal pore fluid pressure, compared with the confining pressure sensitivity, coal permeability sensitivity of back pressure is relatively weak. Back pressure sensitivity experiment can well simulate coal reservoir permeability sensitivity characteristics to the reservoir pressure. Confining pressure sensitivity experiment results would overestimate the damage degree of the actual reservoir permeability.Stress sensitivity is coal’s inherent nature. According to change manner differences of effective stress caused by confining pressure and back pressure, we can obtain that the effective stress coefficient is the ratio of back pressure sensitivity curve regression coefficient and confining pressure sensitivity curve regression coefficient. The effective stress coefficient of undeformed coal calculated is obviously lower than 1. So using Terzaghi effective stress to describe fluid-solid coupling characteristics of coal reservoir will bring bigger error. Coal sensitivity contrast experiments of confining pressure and back pressure provide a new calculating method for the effective stress coefficient.By changing the depressurizing rate of coal sample back pressure to simulate the depressurizing intensity of the actual CBM well bottom-hole pressure, we can see that coal sample permeability decreases with the increase of depressurizing rate of back pressure, showing the sensitivity of coal sample permeability to depressurizing rate, due to the velocity sensitive effect. Based on the similarity theory, we can see that the higher the intensity of actual depressurization is, the more serious the reservoir damage near wellbore will be.(3) The study on dynamic change characteristics of coal reservoir permeability and the seepage model building were carried out. Based on the principle of surface physical chemistry and isothermal gas adsorption theory of coal, the thermodynamic calculation model of coal adsorption expansion strain was derived. We can obtain the improved S-D permeability model by replacing the adsorption expansion volumetric strain term with the derived theoretical model of the expansion strain and considering the general form of effective stress. The effective stress coefficient of coal could be calculated by sensitivity contrast experiments of confining pressure and back pressure. Based on the classical cubic law relation between porosity and permeability and the modified S-D permeability model, the porosity model of main seepage pores was deduced. A kind of porosity determination method of original coal reservoir seepage pores was put forward, namely we can determine the main seepage pore aperture lower limit by the seepage pore fractal scale-free division measured by mercury injection experiment of coal, and then obtain its porosity value.Based on the improved S-D permeability model, permeability dynamic characteristics were analyzed for undeformed coal and deformed coal reservoirs in the target area. Permeabilities of the two coals are all significantly decreased in early depressurization stage, and the stress sensitivity damage of deformed coal reservoir is significantly larger than undeformed coal’s; In the later stage, the effect of matrix shrinkage restrains the decline of permeability, and there is a relatively small rise of permeability in the low-pressure stage. The permeability improvement effect of matrix shrinkage to undeformed coal is significantly greater than that of deformed coal.According to the plane radial flow theory of conventional oil/gas well and considering the evolution characteristics of porosity and permeability of coal reservoir, the mathematical physical models of unsteady seepage flow in the deformation medium coal reservoir for the single phase water and gas were built in the producing process of vertical CBM well.Coal matrix shrinkage effect makes the reservoir has a certain self-adjustment ability, which is a specific poro-perm characteristic of coal reservoir. The simulation results of vertical well production capacity for gas saturated coal reservoir show that, daily gas production is in a slow downward trend as a whole. Daily and accumulative gas productions for coal reservoir of self-regulation effect are bigger than those for coal reservoir of non-self-regulation effect. Because the pore and permeability conditions and self-adjusting ability of deformed coal reservoir are relatively poor, CBM vertical well capacity of undeformed coal reservoir is obviously higher than that of deformed coal reservoir.(4) The characteristics of propagation of pressure drop for coal reservoir in drainage process were studied. The results show that, in the earlier drainage stage of CBM well, reservoir pressure drop longitudinally deepens gradually, meanwhile it also slowly extends outward laterally in the mode of constant depressurization speed. Reservoir pressure drop is significant in a smaller scale near wellbore zone and gradually declines outward. Reservoir near wellbore zone damages seriously along with depressurization. The extended range of pressure drop funnel is very limited. So when the bottom hole pressure reduces to a certain level, drainage mode should be changed to constant bottom hole pressure mode to further increase the extended range of pressure drop funnel.In the constant bottom hole pressure mode, the transmission process of pressure drop could be distinctly divided into two stages, namely, early stage and late stage. In the early stage, the pressure drop funnel has not extended to the outer boundary yet and the propagation speed of pressure drop is relatively fast. The late stage refers to the stage after the pressure drop funnel extending to the outer boundary. The lower the bottom hole pressure is, the greater the pressure drop funnel will be formed during the same drainage time. When the drainage time is relatively long or the bottom hole pressure is relatively low, the influence of the outer boundary condition on the reservoir pressure drop will be greater. Therefore, comprehensively considering the cost and effect of the pressure drop, we should select the reasonable drainage and depressurization system to make the pressure drop efficiency up to the highest.The smaller the differential pressure of production(namely, the higher pressure of downhole) is, the smaller difference of pressure drop expansion in the Y reservoir(Considering the stress sensitivity) and the N reservoir(Regardless of the stress sensitivity) is, then the coal reservoir could be considered as the N reservoir, namely the coal reservoir stress sensitivity could be neglected. The higher the differential pressure of production is(namely, the lower the bottom hole pressure is), the more distinct the coal reservoir stress sensitivity is, and the coal reservoir should be disposed as the Y reservoir. Differential pressure of production is the first critical factor for the effect of stress sensitivity on the pressure drop, and the drainage time is the secondary factor. In addition, the lower the strength of depressurization is, the greater effect of stress sensitivity on the propagation of reservoir pressure drop will be.The bigger the speed of depressurization is, the less the time of depressurization will be, and the smaller the expansion range of the pressure drop funnel will be for whether N reservoir or Y reservoir. This is the general law of the spread of reservoir pressure drop affected by depressurization intensity, which could be thought of as the inherent property of coal reservoir. Its control mechanism could be attributable to Darcy seepage law. Namely, the larger depressurization rate will lead to a larger production differential pressure gradient. In addition, there is a critical speed of depressurization. The selection of actual speed of depressurization should be determined according to the target bottom hole pressure and the critical speed of depressurization.The basic reason for the low productivity of gas well under high production intensity is that the pressure drop funnel expansion is finite, and the gas supply area is small. The effect of drainage intensity on gas productivity is not directly related to the reservoir damage. To the coal reservoirs with low gas saturation, we should slow down the speed of depressurization, so that the pressure drop could reach the most effective expansion in the reservoir before gas production to keep gas well with the largest production capacity.There are four main physical parameters of coal reservoir, such as original porosity(φ0), original permeability(k0), Poisson’s ratio(ν) and fissure volume compression coefficient(cf), which determines the pressure drop spreading. k0 is a parameter which has the strongest sensitivity on pressure drop spreading, and there is a positive correlation between the two. And there are negative correlations between φ0, ν, cf, and pressure drop spreading. Meanwhile, ν shows the weakest sensitivity on pressure drop spreading.In the same drainage conditions, the spreading scale of pressure drop funnel of granulated-mylonitized coal reservoir is smaller than that of undeformed coal, which shows a poor pressure drop spreading capability, and poor original permeability is the major reason. So the effective pressure drop spreading area is very limited in deformed coal reservoir, and gas desorbs difficultly, which makes CBM development difficult and its productivity low.(5) Reservoir pressure drop propagation characteristics were researched in the process of CBM well pattern drainage. Interwell interference effect makes the whole reservoir pressure drop be of large range, quick speed, and more balanced. Well pattern drainage overcomes the single-well drainage limitation of reservoir pressure drop, and is an effective method for improving the productivity of CBM well.Based on reservoir pressure drop characteristics of well pattern drainage, an optimization method of well pattern arrangement was put forward. Namely, define a well pattern optimization index OI to quantitatively describe the total regional pressure drop value under unit drainage cost. Looking for OI peak can determine an optimal well pattern arrangement. Through numerical simulation the optimal well spacing was obtained at corresponding drainage condition, which confirms the feasibility of this well pattern optimization method.