Establishing Water Injection Dynamics by Leading-Edge Coreflood Testing

Abstract

The increasing amounts of water being produced from oilfields, and the increasing need or necessity to return it to the reservoir it ori-ginated from, are posing a challenge to the industry. A fundamental question in Produced Water Reinjection (PWRI) is: "How clean is clean ?", or perhaps even more succinct : "How clean is fit for purpose ?" There is no universal correct answer to this question, as it depends on specific variables, largely intrinsic properties of a reservoir, its produced fluids and the contaminants that eventually end up in the produced water. PWRI for reservoir management purposes must find the balance between injector plugging and the extent of induced fractures [1], for which duty the available simulation models have been found to be wanting as they do not adequately describe leak-off dynamics. Recognizing this constraint, Shell decided that there is a need for unambiguous empirical data that do not suffer from the limitations associated with commercially available coreflood services. A coreflood rig was designed and built, featuring an accurate, automated high injection pressure capability and a much extended test duration. This test rig has since 2007 been field tested at two oilfields, main findings being:-Steady-state conditions can only be achieved in long term exposure windows-Leak-off dynamics at high dP cannot be extrapolated from experiments at low dP-Filtercake permeability depends on the permeability of the flooded core-Membrane tests (Barkman-Davidson) fail to produce representative filtercake properties Although the test rig performed satisfactorily, prospects for further development were identified, leading to a hardware upgrade and quality monitoring instrumentation that is expected to produce even better results in imminent field tests. It is expected that the now gleaned information will provide much improved input for simulators modelling fractured injection. 1. Introduction Injectivity decline is a major issue in most water floods, on a global basis some 80% of injection targets are being met. The 20 % shortfall is largely due to a mismatch between water quality and recipient reservoir. Where targets are being achieved, however, it is often doubtful whether reservoir sweep satisfies projected needs, for the very same (mismatch) reasons. Sub-specification water quali-ty may cause the injected water to predominantly enter highly permeable zones, or create large fractures in unwanted directions. When contemplating PWRI for reservoir management purposes, the cost of achieving a minimum required injection water quality (i.e. complexity of water treatment system) must be weighed against achievable incremental reservoir yield. Establishing these minimum quality needs requires knowledge of leak-off dynamics in an actual injection system. Current thinking is that chemistry and wettability related effects invalidate the traditional view on mechanistic particulate plugging of permeable media. The composite chemical landscape depends on the properties of reservoir rock, injected fluid and its contaminants, but also on additives that are always being used to facilitate key processes. As these interactions cannot easily be modelled, water qualities are being placed in a actualized perspective by resorting to empirical techniques, such as core flood tests. Many core flood tests have been undertaken over the past few decades [2], but unfortunately only few sufficiently approximated, let alone validated actual injection practices. Recognizing this, Shell developed a core flood test rig, designed to provide accurate, fully automated test facilities, capable of con-ducting tests at high injection pressure, at virtually unlimited test duration.

An Analytical Model to Predict the Effects of Suspended Solids in Injected Water on the Oil Displacement Efficiency during Waterflooding

“…Reusing produced water decreases the cost of water disposal and reduces environmental pollution risks [3,4]. Statistically, around 20% of waterflooding projects do not reach planned effects due to the injected water quality and associated formation damage that causes reduction of formation permeability and injectivity [5]. Impairment of permeability was observed in many laboratory experiments and is associated with the plugging of pore space with suspended solids in injection water [6][7][8][9].…”