ABSTRACT:

Tight naturally fractured sandstone reservoir plays an important role in modern oil and gas development. However, it suffers from stress sensitivity damage due to well-developed natural fractures. To understand it, this paper introduced transient pressure pulse decay test (PDP), a time-saving and accurate method, to quantitatively evaluate the stress-dependent permeability, using in-situ core samples with different patterns of natural fractures, and the stress sensitivity index was used to describe the level of permeability recovery. As a consequence, the initial permeability of rocks with partially filled fractures are generally higher than that of rocks with filled fractures, because the partially filled material contributes to the flow by restraining compaction. During the loading process, if the fractures have smaller width, the permeability decreases less. During the unloading process, faster recovery rate requires type A rocks with lower inclination of the fractures. Rocks with micro-fractures have permeability hysteresis, which especially happens to rock samples with the narrowest fracture width, resulting in stronger stress sensitivity effect. This work gives insight to estimate stress-dependent permeability with different natural fracture patterns.

1. Introduction

Tight sandstone reservoirs with natural fractures tend to develop petroleum resources more efficiently and have a favorable prospect especially in the process of the hydraulic fracturing treatment. However, due to low porosity, low permeability, and well-developed natural fractures, tight sandstone reservoirs have strong stress-dependent damage caused by the decreases of formation pressure in the later stage of reservoir stimulation, which can reduce the reservoir permeability and then oil and gas production drops dramatically (Zhang et al., 2019). In the fracturing process, it is inevitable to squeeze the matrix, and such extrusion will lead to an increase in formation pressure. With the mutual influences of overburden pressure increment and porosity pressure decrement, the net pressure increases, leading to the permeability damage, which will negatively change the flow dynamics of the fluids in porous media. This kind of damage is also known as stress sensitivity effect.

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