Abstract
Abstract
Water production from oil and gas reservoirs is increasing world-wide, as more reservoirs are becoming mature. In order to control water production, polymers or gels are injected into production wells to either block the flow, or to reduce water permeability. In the latter case the polymers/gels are known as relative permeability modifiers. In these treatments, a gel is often formed in-situ through a chemical reaction, creating a semi-solid material that is capable of modifying the permeability.
To visualise the processes that occur during the flow of water and oil through a porous rock containing gel, experiments were conducted in transparent glass models in which the flow events can be observed. To form the gel in-situ, a novel alkyl silicate gelant was used, which is soluble in the oil phase, and which, when in contact with water, reacts to convert the water phase into a gel. From the flow observations, the underlying physics can be extracted and the basic mechanisms understood (reduction in effective pore size distribution, gel-bounded water, wetting films of free water, etc.). These mechanisms were used to develop a conceptual model, which consists of three main elements: pore space (bundle of capillary tubes), occupancy of the phases (fluid and gel distribution), and flow equations.
The output from the conceptual model is the capillary pressure as a function of total water saturation and end-point relative permeability. The results from this model successfully represent the disproportionate permeability reduction observed in micromodels and core experiments, verifying the mechanisms included in the model. This model may be quickly and easily used to study the influence of pore size distribution, gelant concentration, and initial water saturation, before undertaking laboratory or more complex modelling studies.
Introduction
High water production in association with crude oil is one of the major production difficulties for the petroleum industry, with water often comprising more than 50% of the produced fluids. Coning due to bottom water drive and production from high permeability, watered-out layers during water flooding are among the main causes. Water handling and disposal costs often shorten the economic life of a well. Disposal of produced water is also an environmental concern, especially offshore. One method to control water production that has been used with some success is to inject gelant solutions into the near well-bore formation. After the gel has been formed, in general, they reduce the permeability to water much more so than to oil; this phenomenon is known as disproportionate permeability reduction (DPR).
The ideal treatment will reduce the water effective permeability without affecting the oil permeability. Although many successful polymer gels experiments and field applications have been reported in the literature, the exact mechanisms by which this disproportionate permeability reduction occurs are not understood. However, as the fluids used before and after treatment remain unchanged, some modifications in the saturation, fluid distribution and flow should occur at the pore level due to the presence of the gel. The mechanisms and causes of DPR could vary with the polymer or gel system and the particular conditions studied. Several mechanisms have been proposed1–6 and verified under certain conditions, but still there is no consensus regarding its validity.
In this study a silicate gel is produced in-situ by using a relatively new gelant system7. This gel system has been proposed as a means to reduce excess water production by treating the production wells, i. e., near-wellbore treatment. As the gelant is oil soluble, it could be injected into the formation without precipitation. Therefore, this chemical system should not damage the oil producing zones. During shut-in period the reaction takes place converting the water into gel while ensuring oil continuity.