Abstract

It is a common occurrence worldwide to have hole washouts while drilling with water-based mud. There are several reasons for this and until a concerted effort is taken to address the issue, such holes are bound to develop. Some of the problems that large washed-out holes cause are poor hole cleaning while drilling the hole, increased chances of stuck pipe, poor wireline log quality, bad zonal isolation after cementing primary casing strings, and loss of production due to inadequate zonal isolation.

This paper discusses custom-designing water-based systems to address the problem of washed-out holes and wellbore stability. It details the laboratory testing performed on cores and drill cutting samples from the drilled intervals to assess the suitability of appropriate chemicals. The primary study concentrated on understanding the shale inhibition process using water-based fluids. The mineralogical compositions of the shales from the drilled areas were analyzed and tested with various base fluid types and additives.

The recommendations generated from these studies were implemented in high temperature wells drilled by this operator. The results obtained from drilling these wells have been reduced hole wash-outs and stable wellbores.

According to conservative estimates, the problems resulting from shale instability costs the petroleum industry over half a billion dollars per year. The learnings from this paper should help other operators overcome such issues and cut down on non-productive time and expenditures.

Introduction

The wells drilled in the Western Desert of Egypt primarily target the Cretaceous and Jurassic age formations. Khalda Petroleum Company operates on concession acreage of approximately 12,500 km2 (Fig 1). The geological structure in the Western Desert is generally uniform throughout the operating areas, consisting mainly of shales, limestone and siltstone sections. Statistically, shales account for 75% of the drilled sections and cause 90% of the welIbore instability-related problems.1 The majority of the washouts occur for this operator in the shale sections of the Abu Roash (A/R) formation and Alam El Bueib (AEB) formations (Fig 2). These two different hole sections are drilled in the drilling sequence.

The Abu Roash (A/R) formation starts from an approximate depth of 4,000 ft and has a thickness of 2,500 ft. The formation consists of more than seven members ranging from A/R A to G. The lithology alternates between shale bodies to siltstone and limestone sections.

The Alam El Bueib (AEB) formation starts from an approximate depth of 9,100 ft with a thickness of approximately 3,500 ft consisting of more than eleven members ranging from AEB #1A, 1B broken into further subdivisions until AEB #6. Once more the AEB formation lithology alternates between shale bodies and siltstone and dolomite sections.

Understanding Hole Washouts in the Western Desert
Formation type.

By nature, shales are known to be chemically active. They are fine-grained sedimentary rocks composed primarily of clays, with parts of silt and in some cases fine sands. Depending on their composition, some clays can be extremely sensitive to water and water-based fluids. Water-sensitive clays undergo an ion-exchange process with the salts present (or absent) in the water resulting in an imbalance in its ionic stability 2 leading to clay swelling, clay migration and even breakdown of the rock structure.

Sufficient confining pressure to hold up the rock helps keep shales in place (i.e., hydrostatic pressure). However, after drilling out the formation the in-situ stresses that keep the rock intact are altered. This in conjunction with a change in the ionic balance of the clay (due to the fluid exposure) induces mechanical weakness in the formation. The exposure process is of course time dependent and as the rock is exposed to more fluid with time the mechanical property of the formation deteriorates further.

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