Abstract

Water injectivity decline is a very common phenomenon in waterflooding fields. Most of the previous analyses were focusing on water injectivity decline due to the migration of suspended particles in injection water or the injection water/reservoir fluid incompatibility. However, in some unconsolidated formations, another possible mechanism for water injectivity decline is sand mobilization, which means sand particulates separate from rock matrix and move into deep formation. This kind of injectivity decline is controlled by the operation condition of water injection such as injection pressure and injection rate. In this paper, a mathematical model is proposed to simulate the process of sand mobilization and the resultant water injectivity decline.

The mathematical model is derived based on material balance for water, sand particulate, and rock matrix. Also included in this model are particulate generation and deposition constitutive laws, and permeability-porosity correlation. Finite difference scheme is introduced to discretize the partial differential equations and the finite difference equations are solved implicitly through iteration. Sensitivity analysis is performed to study the effects of various factors on water injectivity decline and strategies for managing efficient water injection are proposed through the analysis.

Introduction

Numerous researches[1–13] have been done in oil/gas well sand production. Because water injectors are not generally back produced and as a result very few researches were done in the past on injector sanding. Literature survey indicates that only a couple of papers[14–15] were published regarding this topic. Despite that, it does not mean sanding in water injectors is not a problem, instead, it can cause the injectivity decreases dramatically. As stated in reference[14], the injectivity of a well operated by Statoil in the Norwegian Sea decreased from 8000 m3/d to 0 m3/d in just half an hour which is tied to formation failure caused by the pressure waves generated during the sudden shut down of the pumps. From this single example, we can see that how bad it can be in water injectors once sanding occurs. Because of this, researches on sanding in water injectors are of the same importance as those in producers.

Santarelli et al[14] presented a field case study on a reservoir operated by Statoil in the Norweigian Sea concerning the injectivity decline of water injectors. In this paper, it is believed that sanding is caused by the following reasons:

  1. During well shut-in, the rock around the well is too weak to sustain the stresses and fails.

  2. Because of the reservoir permeability heterogeneity, the wells are cross-flowing during shut-in and cause sand production in front of the perforated intervals.

  3. The produced sand is not able settle down in the rat-hole before injection restarts and hence plugs the perforation tunnel.

  4. As a result of the water hammer effect caused by well shut-in, the formation already weakened by sand production undergoes liquefaction that triggers large amounts of sand to be released in the well and hence killed the injectivity. Morita et al[15] provided guidelines for completing water injection wells.

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