Analytical modelling of the water block phenomenon in oil and gas wells

Abstract

In oil or gas production wells, water can become trapped by capillary forces in the reservoir rock near to the wellbore or hydraulic fracture face. The trapped water dramatically reduces the production rate of oil or gas. The general term for wells suffering from productivity loss due to capillary trapped water is “water block”. The severity of the water block is the result of the combined effects of wettability, capillary forces, viscous forces, fluid compressibility, hydraulic fracture length and various other reservoir and well properties. A numerical model can be used to evaluate all the effects for particular scenarios. However, for most oil or gas reservoirs, there is great uncertainty in many of the reservoir parameters. Evaluating enough scenarios numerically to account for all the uncertainty can be extremely time-consuming. Analysing the sensitivity to uncertain parameters is substantially faster and better served by analytical models. Historically, the impact of the end effects in production wells have been neglected in analytical models as the target reservoirs have generally had high permeability. Now, the development of petroleum fields involves targeting much lower permeability reservoirs. These reservoirs can exhibit high capillary pressures. Despite the significant capillary pressure effects, analytical models have yet to be developed for gas or oil well performance accounting for capillary end effects. The goal of this thesis is to develop analytical models to include the effects of capillary forces near to the wellbore or the hydraulic fracture and to analyse the impact of fluid compressibility, wettability, capillary forces, viscous forces, porous media network characteristics and hydraulic fracture half-length. This thesis includes five journal papers, four of which have been published; the last one finished with intention to submit later this year. The thesis develops new analytical models to calculate the productivity index of gas wells. The new models account for viscous forces, capillary forces, inertial forces, wettability, compressibility, non-uniform flow into hydraulic fractures and capillary end effects near to the wellbore or hydraulic fracture. The analytical models are validated through matching with experimental data or numerical simulators. The forms of the capillary pressure and relative permeability curves are extremely important properties of the reservoir which impact the magnitude of the capillary end effect. The shape of the curves is significantly impacted by the characteristics of the pore network. A sophisticated mixed percolation model coupled with effective medium theory is developed in the thesis. The novelty of the new percolation model is that it applies mixed bond-site percolation for the first time to the water-hydrocarbon drainage problem, while only bond or site percolation was applied in previous models. This thesis integrates the complex interaction between the viscous, capillary and inertial forces, water cut, hydraulic fracture length and the topology of the pore network. The study quantifies the impact of each of the aforementioned characteristics of two phase flow on the productivity of gas wells. The results of this thesis are critical in screening for economic well candidates for intervention.