Abstract
The inter-salt argillaceous dolomite reservoirs in the central region of China contain large abundance of oil resources with ultra-low permeability and porosity. However, the oil wells in this area show a very quick reduction with the decline of formation pressure. This article aims to investigate the main possible reasons that affect oil well productivity in the target oilfield. This study begins with analysis of capillary microscopic model, core stress sensitivity experiments, and non-Darcy percolation experiments. The impact of effective stress on permeability and porosity of the reservoir was revealed in this article. The novel productivity model and productivity evaluation model which couples stress sensitivity and threshold pressure gradient were proposed. The analysis of capillary microscopic model shows stress sensitivity of permeability to be much greater than that of porosity during the process of depressurization. The core stress sensitivity experiments results indicate that permeability and effective stress show index relationship while porosity and effective stress show binomial relationship. Damage rate and recovery rate of permeability and porosity were put forward to describe the degree of influence of stress sensitivity on permeability and porosity. The models were used to investigate the factors that affect single well productivity for the target oilfield. Application of the proposed model to this tight oilfield indicates that, the degree of influence of stress sensitivity is much greater than that of threshold pressure gradient. In addition, the greater the stress sensitivity coefficient and threshold pressure gradient are, the greater the productivity reduction will be.
Highlights
The stress sensitivity of reservoir rock is that, its permeability and porosity change when the effective stress acting on it changes
The aims of this article are (1) variation rules of porosity and permeability based on core stress sensitivity experiments for target reservoir, (2) to establish the productivity model and productivity evaluation model that considered stress sensitivity and threshold pressure gradient for the target reservoir, and (3) investigate the factors that affect single well productivity for the target oilfield
The reason why permeability stress sensitivity is far greater than porosity stress sensitivity during the development process of depressurization was elaborated
Summary
The stress sensitivity of reservoir rock is that, its permeability and porosity change when the effective stress acting on it changes. Fatt and Davis (1952) studied permeability stress sensitivity and they found out that, permeability reduction ranges from 11% to 41% when the effective stress acting on it changes. McLatchie et al (1952) used oil to study the stress sensitivity of cores whose permeability ranges from 3 to 102 Â 10À3um. McLatchie et al (1952) used oil to study the stress sensitivity of cores whose permeability ranges from 3 to 102 Â 10À3um2 Their experimental results suggested that the irreversible reduction for permeability is 4% in high permeability cores while it is 60% in low-permeability cores. Su et al (2000), Ruan et al (2002), Li (2006), and Jiao et al, (2011) studied permeability stress sensitivity of different low-permeability rocks. Their research results indicated that the lower the permeability of the rocks, the stronger the permeability stress sensitivity of the reservoir rock will be. Liao et al (2012), He et al (2012), and Li et al (2013) considered that interstitial materials support throats and they greatly affect the strain of throats and permeability stress sensitivity
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