Abstract

It is widely known that formation damage during drilling and completions are of prime concern for production wells. Formation damage impacts in injector wells, however, have not received the same level of detail and understanding. Water injection wells are of major importance in hydrocarbon recovery. Water injection wells are often completed as open holes, therefore, mud cake clean-up and damage removal by production of formation fluids is often performed in order to maximise subsequent water injectivity. In some wells, however, this production phase can be prevented due to surface handling limitations or low reservoir pressure. In most cases, when a well cannot be backflowed for long periods, it is possible to bypass near wellbore damage by exceeding the fracture pressure, but this may strongly affect sweep efficiency and uniform injection. Consequently, it is often necessary to inject water immediately after drilling and completion i.e. no possible production phase. In this case, a complete well clean-up is required, including the removal of the majority of mud filter cake components in order to maximise water injectivity. The objective of the present study was to develop a completion fluid formulation, based on a previous breaker technology developed by IFP, that allows complete disruption of the oil based mud cake by coalescence of water droplets and dispersion/dissolution of solids which is also compatible with all drilling and production fluids. The experimental methodology used in this study included several steps: the screening of efficient water based additives for dispersing/dissolving solids, the formulation of water-in-oil emulsions with controlled delayed kinetics of breakage adapted to the planned completion program, the assessment of the formulation compatibility with production fluids and finally the validation of clean-up efficiency with dynamic core filtration tests with a significant injectivity enhancement.

Introduction

In most cases, injection wells are completed as open holes to obtain higher water injectivities. Consequently, it is recommended to produce the well prior to injection in order to clean-up the mud filtercake and to remove damage. When the period of backflow is not feasible or desirable, due to surface handling limitations or low reservoir pressure, it is possible to bypass near wellbore damage by injecting water above the fracture pressure. But this operation may strongly affect sweep efficiency and uniform injection or is sometimes not feasible due to pump limitations. Consequently, it is often necessary to inject water immediately after drilling and completion without performing any backflow and without exceeding frac pressure.

In this case, a full well clean-up is required, including the removal of the majority of mud filter cake components (fluids, polymers and solids) in order to decrease damage and maximise water injectivity.

It is noteworthy that while many papers can be found in the literature on mud cake cleanup in producer wells 1–6, few papers are dedicated to cleanup of injector wells 7, 8. In a previous paper 8, a series of experimental results relevant to water injection wells were presented and clearly showed that high injectivities may be achieved without any prior production using several types of drilling fluids (water-based or oil-based). A three-steps cleanup process showing tremendous benefits in retained injection permeabilities was recommended.

The objective of this paper is to present a new fluid formulation, that allows a simple one-step cleaning procedure for water injection wells drilled with oil-based muds. This formulation is a weak water-in-oil (w/o) emulsion with controlled delayed kinetics of breakage that allows complete disruption of the oil based mud cake by coalescence of water droplets of the filter cake followed by dispersion/dissolution of solids. Most important, it is also compatible with all drilling and reservoir fluids. The composition of the continuous oil phase of this formulation is based on a previous breaker technology developed by IFP for cleaning producer wells drilled with Oil-Based Muds (OBM)5. The main mechanism of action of this oil-based breaker consists in weakening the OBM cake structure by initiating and speeding up the coalescence between brine droplets. Then, the water-based breakers added in the dispersed aqueous phase of the proposed formulation may play their role of solids dissolver, hence allowing the removal of the cake.

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