Abstract
Stress sensitivity in reservoirs is critical during the exploitation of oil and gas fields. As a deep clastic reservoir under strong tectonic compression, the Ahe Formation in the northern tectonic zone of the Kuqa depression exhibited strong stress sensitivity effect. However, the conventional evaluation method by using permeability damage rate as a constraint restricts the mechanistic understanding of the strong stress sensitivity effect. In this study, morphology of stress sensitivity test curve, coupled with rate change of permeability and extent of irreversible damage in actual sample measurement through micro-CT in-situ scanning, is used to characterize differentially. The strong stress sensitivity effects of the studied intervals can be divided into three types: (1) rapid change in permeability–weak irreversible damage, (2) moderate change in permeability–strong irreversible damage and (3) moderate change in permeability–moderate irreversible damage. The strong stress sensitivity is caused by the micro-pores and micro-fractures, which are widely developed in the studied reservoir. The mechanisms caused by the two types of pore are different. The stress sensitivity effects in micro-fracture-rich reservoirs are characterized by rapid change in permeability and weak irreversible damage. Meanwhile, the stress sensitivity effects in micro-pore-rich reservoirs are manifested as moderate change in permeability and strong irreversible damage. The study shows that the differences in the content of micro-pores and micro-fractures and their reverse mechanisms of stress sensitivity co-create different types of stress sensitivity within the samples. Accordingly, the differences of the stress sensitivity type in macroscopic samples are caused by the competition between the microscopic differences of pore types.
Highlights
Stress sensitivity in reservoirs is critical during the exploitation of oil and gas fields
In order to make the testing samples as diverse as possible in mineral composition and pore-throat structure characteristics, 13 representative samples from the Ahe Formation were collected from 10 wells in the northern structural belt, the Kuqa depression (Fig. 1)
With the confining pressure releasing in the later period, the permeability of the samples increase gradually, resulting in low degree of irreversible damage. This type is characterized by rapid stress sensitivity change and weak irreversible damage
Summary
In order to make the testing samples as diverse as possible in mineral composition and pore-throat structure characteristics, 13 representative samples from the Ahe Formation were collected from 10 wells in the northern structural belt, the Kuqa depression (Fig. 1). As the confining pressure increases continuously, the difference in the change rate of permeability (the absolute value of the slope of the test curve) is roughly divided into two segments, namely the change rate 1 and the change rate 2 (Table 1, Fig. 5). The micro-pores that are initially squeezed are difficult to recover due to its plasticity, so that the reduced permeability of the sample is difficult to reverse, showing a relative strong irreversible damage rate. Both pore types can restrict stress sensitivity effect of the samples significantly, and two kinds of sensitization mechanisms can illustrate their exist significant difference. The stress sensitivity effect is the macroscopic result of the combination of the two different mechanisms above
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