Downhole Monitoring of Fractures in a Waterflood Development – Part 1

Abstract

When an oilfield is exploited by simply producing oil and gas from a number of wells, the reservoir pressure in many circumstances drops quicker than normal impacting the production rates (Koning, 1988) and well performance. To maintain the pressures in the oil producing formations, waterflooding enhancement method is implemented by the Operators. This is achieved by drilling injection wells or converting the oil producing wells into injectors. The injection wells are located at carefully selected points in the oilfield so that the water displaces as much oil as possible to the production wells before the water starts to break through. A significant saving in an oilfield development can be obtained by reducing the actual number of injecting wells and increasing each of the injector wells' capacity for injection. Balancing the injection and produced volumes often involves injecting at high pressures leading to the fracture of the reservoir rocks along a plane intersecting the wellbore. This happens when injection pressure overcomes the rock stress and its tensile strength, thereby creating an induced fracture network. With continuous injection, these fractures start propagating into the reservoir and may reach the reservoir caprock. Continuing to inject further in such a fracture system may breach the top seal integrity of the caprock leading to uncontrolled out of zone injection. The study of evaluation of downhole fracture monitoring is divided into two parts. In this paper a downhole verification approach to identify the fracture initiation point(s) is the focus. It describes the planning, execution and interpretation of the downhole data. This includes spectral acoustic monitoring and modelling of the temperature responses to quantify the injectivity profile. In paper (Kohli, Kelder, Volkov, Castelijns, & van Eijs, 2021), the direct business impact and regulatory requirements are discussed by further integration of acoustic monitoring and temperature modeling data with detailed results from downhole measurements of fracture dimensions by means of pressure fall off tests. Combined, both studies form the integrated approach that the Operator took to meet the regulatory requirements proving that the fracture network propagation remains within the reservoir and that the top seal integrity is maintained.