Production performance of the low-permeability reservoirs: Impact of contamination at the wellbore vicinity

Abstract

Hydrocarbon reservoirs with low porosity and permeability are usually vulnerable to be contaminated owing to the drilling and completion fluids-induced formation damage, which significantly impedes the efficient production of hydrocarbon. However, most current productivity prediction models introduce the concept of skin factor to capture the near-wellbore contamination, which only considers the change of permeability before and after contamination, whereas neglecting the change of threshold pressure gradient (TPG) affected by the permeability reduction. Also, few works have been done to focus on the influences of contamination, TPG, sensitivity of rock porosity to internal stress, and fluid compressibility simultaneously in the actual production process of the low-permeability reservoir. Therefore, to bridge the knowledge gap, this study presents a new comprehensive productivity model encompassing those multiple influencing factors. Firstly, the seepage area is divided into contaminated zone and uncontaminated zone. Thereafter, the proposed model is established based on non-Darcy seepage theory and continuity principle, comprehensively taking account of the effects of different permeability and TPG between contaminated zone and uncontaminated zone, stress sensitivity based on primary stress in porous media, and the fluid compressibility. Subsequently, the newly proposed model is proved to be reliable through the consistency between the simplified model and the existing models in the literatures. Finally, the sensitivity analysis is employed to investigate the effects of key parameters. Main results show that a) the decreased permeability affected by contamination would result in increased TPG, further lowering the well productivity. b) Contamination, TPG and stress sensitivity will cause additional pressure drop and production loss, which will overestimate the production rate by neglecting them. While the fluid compressibility could boost the well productivity, which will underestimate the production rate by ignoring it. c) The order of productivity influencing factors is contamination > TPG > the fluid compressibility > stress sensitivity. d) Contamination and the TPG are significant influencing factors at stage of high bottomhole pressure. Moreover, the influence degree of contamination, the fluid compressibility and stress sensitivity gradually increase, while the influence degree of the TPG greatly decreases with lower pressure.