Summary

Dongping Field with low porosity and permeability, located in the Qaidam Basin, is the largest bedrock reservoir in China. Because of its high content of clay minerals and poor physical properties, the reservoir tends to suffer from formation damage caused by water or alkali sensitivity, which significantly affects production. Therefore, it is crucial to understand the formation‐damage mechanisms for reservoir protection and efficient development. In this paper, we propose a new method that integrates pressure‐transmission tests (PTTs) with a fixed‐point scanning electron microscope (SEM) and nuclear magnetic resonance (NMR) to investigate mechanisms of formation damage. From the macroscopic perspective, PTT, a time‐saving and accurate method of measuring permeability for tight rocks, is applied to evaluate the permeability‐damage degree of core samples. From the microscopic perspective, the change of pore structure for a specific region and the variation of the fluid volume in the core sample before and after damage are obtained by fixed‐point SEM and NMR experiments, respectively. In water‐sensitivity damage experiments, the results of PTT show that the permeability‐damage rate is approximately 60%, demonstrating that the damage degree of water sensitivity is moderate to severe. The results of the fixed‐point SEM demonstrate that the pore sizes suffer from obvious reduction as a result of water‐sensitivity damage. NMR experiments demonstrate that the main damage mechanisms vary, depending on the pore sizes. Small pores [0.1 milliseconds ≤ transverse relaxation time (T2) < 10 milliseconds] are damaged by swelling of mixed‐layer illite/smectite, and the large pores (10 milliseconds ≤ T2 <100 milliseconds) suffer from the combined damage of swelling of smectite and migration of illite. Using a similar method, the alkali‐sensitivity damage test demonstrates that there is no alkali‐sensitivity damage in target reservoirs. This novel integrated method is an efficient means to investigate the formation‐damage mechanisms for the Dongping bedrock reservoir from comprehensive perspectives. In addition, this method can be widely applied to evaluate formation damage for tight low‐permeability reservoirs.

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