An inversion method of initial coal reservoir pressure using fracturing process data

SHI Juntai; LI Wenbin; ZHANG Longlong; JI Changjiang; LI Guofu; ZHANG Sui'an

doi:10.13809/j.cnki.cn32-1825/te.2022.04.003

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An inversion method of initial coal reservoir pressure using fracturing process data
Issue 4, Pages: 564-571(2022)
Published：2022
DOI： 10.13809/j.cnki.cn32-1825/te.2022.04.003
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DOI：

SHI Juntai, LI Wenbin, ZHANG Longlong, et al. An inversion method of initial coal reservoir pressure using fracturing process data[J]. Petroleum Reservoir Evaluation and Development, 2022, (4): 564-571. DOI： 10.13809/j.cnki.cn32-1825/te.2022.04.003.

摘要

原始煤储层压力是煤层气储量和产能评价中一个非常重要的参数，对煤层气开采具有指导作用，准确计算原始煤储层压力具有重大意义。因此，基于渗流力学理论压力势叠加原理，建立了压裂过程中和压裂后关井过程中压力势模型，并分忽略与考虑煤储层压裂过程中和压裂后裂缝网络渗透率变化两种情况，提出了利用水力压裂煤层气井关井阶段井口压力降落数据反演原始煤储层压力的方法，并进行了实例应用。从实例井拟合结果可以看出，忽略与考虑煤储层裂缝网络渗透率变化的原始地层压力方法线性关系都很明显，说明建立的方法有效。从解释精度的角度来看，考虑煤储层压裂后裂缝网络渗透率变化的方法参与拟合的数据点更多，且压裂过程和压裂后煤储层裂缝网络渗透率变化是不争的事实，解释结果更加可靠。如果忽略压后关井期间煤层裂缝网络渗透率的变化过程，评价的裂缝网络稳定的渗透率及原始煤储层压力会偏高。该方法在确定原始煤储层压力的同时，还可以确定煤储层压裂后裂缝网络稳定的渗透率并评价裂缝网络渗透率变化趋势，为煤层气藏类型划分、煤层气储量计算、压裂效果评价和排采制度优化设计提供依据。

Abstract

The initial formation pressure of coalbed methane（CBM） reservoir is a key parameter used for the evaluation of CBM reserves and productivity

which plays a guiding role in CBM production. Therefore

it is of great significance to accurately calculate the initial reservoir pressure of CBM reservoir. Based on the pressure potential superposition principle of the seepage mechanics theory

the pressure potential model in the process of fracturing and shut in after fracturing is established

and a method for determining the initial reservoir pressure of CBM reservoir is proposed by using the wellhead pressure drop data in the shut-in stage of hydraulically fractured gas well under two conditions: ignoring or considering the fracture network permeability change in the process of fracturing and shut in after fracturing. And then

a field application is carried out. From the fitting results of the case study

it can be seen that the linear relationships for both methods are obvious

indicating that the established methods are effective. From the perspective of interpretation accuracy

the method of considering the fracture network permeability change after fracturing involves more data points in fitting

and the fracture network permeability change during fracturing and after fracturing is an indisputable fact

so the interpretation result by using the method considering the fracture network permeability change is more reliable. If the change process of fracture network permeability of coal formation during shut in after fracturing is ignored

the interpreted stable permeability of fracture network and initial reservoir pressure will be high. Since this method can not only be used to determine the initial reservoir pressure

but also to determine the stable permeability of fracture network after fracturing and evaluate the change trend of fracture network permeability

it provides a basis for the classification of CBM reservoir types

CBM reserve calculation

fracturing effect evaluation

and optimization design of drainage and production system.