Shunli He

The Effect of Stress and Pore Pressure on Formation Permeability of Ultra-Low-Permeability Reservoir

Abstract Low-permeability reservoirs have attracted increased attention from most oil companies due to increased demand and limited reserves in conventional reservoirs. Most scholars agree that worldwide hydrocarbon production decline from conventional reservoirs will be compensated for by the development of low-permeability reservoirs to satisfy growing demand. During the production life cycle of a low-permeability reservoir, permeability at any given location may change in response to the change of stress due to depressurization. Waterflooding is an important method used to develop low-permeability oil reservoirs. Therefore, it is necessary to study whether and how a stress state change can affect formation permeability. However, it is seldom seen the relevant research and report. This article presents a laboratory procedure to simulate the stress state change of ultra-low-permeability formation during injection and production applying cores from Changqing oilfield and evaluate the effect of stress on formation permeability under reservoir conditions using brine as the fluid medium. This work focuses on how formation permeability changes with stress change during injection and drawdown and discusses the difference in permeability change between injection and production. The analysis is not only on the basis of selecting zero pore pressure but also in situ pressure as the starting point; that is, a reference point. The results indicate that reservoir permeability changes during both drawdown and injection are caused by stress variation. Microcracks are the main factor resulting in this permeability change. This experimental method simulated formation stress state and flow behavior as truly. The results are meaningful for analyzing oil well production and reservoir development for ultra-low-permeability reservoir.

Distribution of sulfur deposition near a wellbore in a sour gas reservoir

Elemental sulfur precipitates from sour gas when reservoir pressure and temperature decrease. Sulfur deposition in a formation may significantly reduce the inflow performance of sour gas wells. This paper develops a micro-mechanical migration model and experiments which describe the law of sulfur precipitation, plugging and distribution near a wellbore. Based on the analysis of the sulfur deposition law and micro-mechanical migration theory, elemental sulfur mechanical models in pores are presented. The critical velocity of sulfur is calculated and the rule of precipitated sulfur distribution near a wellbore is deduced. Reservoir cores and supersaturated sour gas are utilized to observe sulfur precipitation and plugging in sulfur damage experiments, and the main influential factor is analysed. According to the models and experimental results, precipitated sulfur can decrease reservoir permeability. The liquid bridge force is the most important factor to affect reservoir permeability due to sulfur deposition. Precipitated sulfur cannot be carried away from pores if the liquid bridge force is considered; the critical velocity increases as the diameter of the sulfur particles increases, which may cause serious formation damage. Moreover, it can be seen from the results that the biggest volume of sulfur deposition does not occur at the bottom but near the bottom of a borehole. These results can be used to describe the profile of dynamic sulfur deposition and help a reservoir engineer to develop a plan for removing the sulfur near a wellbore.

A Deliverability Equation in the Presence of Sulfur Deposition

Precipitated elemental sulfur may block pores, decrease gas flow, and affect sour gas well production if elemental sulfur cannot be carried by gas. A percolation model in the presence of sulfur deposition was analyzed under pseudo-steady state flow. On the basis of fundamental filtration theory, a composite deliverability test model was established to identify the effect of deposited sulfur on open-flow capacity and additional skin. A group of field case was used to calculate and analyze the effect of sulfur deposition on deliverability equation. The results show that gas flow capacity decreases as sulfur saturation rises, while open-flow capacity decreases as damaged radius increases. Also the more volume of deposited sulfur there is, the larger the additional skin is, and the more serious the damage to the reservoir is.

An improved particles model for stress sensitivity in low-permeability sandstones

Abstract Stress-dependent permeability and porosity of porous media is very important for the development of oil reservoirs, especially in low-permeability reservoirs. Porous structure can change in response to an increase in the effective stress to induce the decline of permeability and porosity, which ultimately affects the fluid flow property. In this study, a capillary module, considering the arrangement and the deformation of particles, has been established to express the stress sensitivity of porous media based on the particle packing model. The predictions of variation of permeability by the proposed model have been validated by comparing with the experimental data. The predictions by the current model show the same variation trend with the experimental data and are consistent with the compression experiments. The proposed variation of permeability with the increase in effective stress depends on the factors such as the arrangement of particles (contact angle), the deformation of particles (elastic module and Poisson’s ratio) and effective stress. The effects of these parameters on variation of permeability are discussed in detail.

Improving Decline-curve Analysis of Low-permeability Gas Wells Using Type Curves

The authors present a rigorous method to analyze and interpret production rate and pressure data of low-permeability gas wells using type curves. The method is based on the type curve analysis method presented by Blasingame et al. (1991), but imports the wellbore storage coefficient, which influences the flow behavior of low-permeability gas wells significantly, for the first time. This work fills a significant void in the inventory of decline curve type curves, and the method is applicable to production analysis for cases taken from low-permeability gas wells.

Research and Application of Deconvolution in Well Test Analysis of Extra-low Permeability Sandstone Reservoirs

Abstract For extra-low permeability sandstone reservoirs, traditional well test analysis techniques are always limited by wellbore storage coefficient and the low flow velocity in the porous media. Results derived using traditional well test analysis techniques are meaningless or useless. A deconvolution method based on the newly robust solution algorithms is applied for extra-low permeability sandstone reservoirs. Deconvolution codes were developed based on von Schroeter deconvolution algorithm and Levitan deconvolution algorithm. Though the study is based on synthetic cases and a field case, it is proved that the deconvolution algorithm works well in well test analysis of extra-low permeability sandstone reservoirs.

Notice of Retraction The research of sour gas reservoir damage mechanism

Sulfur deposition damage is one of the most important problems in sour gas reservoir development. With decreasing pressure and temperature near bore zones, solid elemental sulfur may deposit and block pore. Whats worse, it reduces the inflow performance of sour-gas wells. Based on the comprehensive analysis of deposit and block process, solid elemental sulfur motional and mechanical model were established with irreducible water considered. The results indicated that precipitated solid elemental sulfur did not move a long time in pore; they could in-suit deposit. Irreducible water plays an important role in sulfur deposition. The former precipitated solid blocked pore and the latter solid could be carried if gas velocity was big enough.

A Sulfur Plugging Experiment in the Presence of Ferric Ion

Abstract Acid fracturing is widely used as an effective reconstruction measure in sour gas reservoirs, but sulfur and ferrous sulfide plugging are still problems in reservoir secondary damage. In order to simulate reservoir damage in the presence of ferric ion in residual acid, an artificial core with ferric ion and supersaturated natural sour gas was adopted as a sample. A depletion-type damage experiment was carried out to observe sulfur precipitation and plugging in the core. The results show that core color became dark and weight was increased. Permeability decreased with increased time and the decreasing trend was steep at the beginning and then down to smooth rate slowly. Trace pyrite was discovered using X-diffraction. Severe damage was formed by sulfur deposition in the presence of ferric ion.

Effects of stress-dependent permeability on well performance of ultra-low permeability oil reservoir in China

An experimental investigation of the behaviors of stress-dependent permeability under in situ conditions was conducted and discussed, applying cores from an ultra-low permeability oil reservoir in China. The variation characteristics of formation permeability resulting from pore pressure drawdown and increase were compared. The results indicate that formation permeability at any possible location of the reservoir could be altered in response to the change in stress state caused by both oil production and water injection. A mathematical model of fluid flow in stress-sensitive reservoir was established to evaluate the effect of stress changes on well performances, and an analytical solution method was presented. Several analytical simulations under the conditions of constant wellbore flowing pressure were performed to quantitatively assess the impact of stress sensitivity on single well performance. It is demonstrated that despite the stress-dependent permeability can have an adverse impact on production rate and recovery volume, it may be favorable for water injection. Based on the analysis, a practical and efficient waterflooding program was presented to reduce the influence of permeability damage on reservoir productivity. This program was verified by numerical reservoir simulation to have a combined positive effect for development of ultra-low permeability oil reservoir.

Deliverability equation in pseudo-steady state for fractured vertical well in tight gas

Based on non-Darcy flow theory in tight gas reservoirs, a new deliverability equation of fractured vertical gas well in pseudo-steady state is presented with the consideration of the stress-sensitive effect, and the open flow capacity calculation formula of gas well has been also derived. With the new deliverability equation, the effects of stress-sensitive coefficient, fracture parameters and matrix permeability on the productivity of gas well have been analyzed. The computation across an instance shows that due to the stress-sensitive effect, the IPR curves bend over to the pressure axis and the productivity of gas well is lower than that derived from the equation without consideration of stress-sensitive effect under the same pressure drop. As the stress-sensitive coefficient increasing, the well productivity becomes lower, the decline rate of production is higher and the IPR curve bends over in earlier stage with a greater bending. Besides, the productivity is affected by and has a positive correlation with the length and conductivity of fracture, namely that it becomes lower as the length and conductivity of fracture decreasing. Matrix permeability has an apparent impact on the productivity. If matrix permeability is extremely low, gas well cannot achieve the industrial production even after fracturing. As the matrix permeability increasing, stimulation results are significant.