Abstract

For years the drilling industry has wrestled with the problems associated with well cementing. The introduction of pneumatic bulk systems increased cement slurry mixing rates but accurate density control still eluded the experts. Many companies have introduced process controlled mixing systems but these are both expensive and not totally reliable.

A new cementing system, that does not rely on bulking, has become available to the industry - a storable oilwell cement slurry that can be kept in a liquid state indefinitely and made to set as and when required.

Operators in Indonesia are now benefiting from this new technology. Slurries are mixed and tested to ensure they meet design specifications well before they are required on site. The "Base Slurry" or "Liquid Cement Premix", which typically has a density of 16.0 pounds per gallon (ppg), is transported to the rig, diluted to produce the required density and pumped ensuring a homogeneous slurry from start to finish.

Introduction

Due to the staggering economic and population growth in Indonesia the government has undertaken an ambitious plan to supply sufficient electricity to support this development. Situated on the "Pacific Rim", in an area renowned for being part of "the Ring of Fire", geothermal electricity generation is an obvious choice and a crucial component to the success of this plan.

A promising geothermal concession earmarked in the plan is situated in a tea plantation on the island of Java. A drilling project was undertaken to evaluate and harness this excellent resource. The evaluation program incorporated the use of two slim hole rigs to drill appraisal wells and establish the full potential of the field.

Companies are being faced with environmental concerns throughout the world. This, and the requirement to reduce the size of drilling sites in environmentally sensitive areas, necessitated a change in the way these operations were serviced (Figure 1). Cementing companies historically have needed a large work area or "foot print" for bulking and mixing equipment.

Geothermal wells arguably present the most severe conditions to which cements are exposed. As a result, the performance requirements are among the most stringent. At present, geothermal cements are usually designed to provide at least 1,000 psi (7.0 Mpa) compressive strength, and no more than 1.0 md water permeability (1). Meeting these requirements was further complicated with the cement slurry for the 4-1/2" casing where, if lost circulation was experienced during drilling and a coring string utilized to reach casing point, the open hole section could have a diameter as small as 5". While casings with tight annular clearances require that "good cementing practices" be observed (i.e. centralization, drilling fluid conditioning etc.) it also creates conditions that demand much greater care and control in slurry and procedure design than "regular" casing cementations. Guidelines, when designing a cement system for slim-holes specify:

  1. Keeping the cement slurry density fluctuations to a minimum.

  2. Designing the cement slurry to have the lowest practical rheologies while maintaining zero free water breakout.

  3. Enhanced slurry rheologies for optimum wellbore flow phenomena.

While free water and thickening time requirements were similar under geothermal and slim-hole conditions, the use of Perlite and silica flour, in large percentages, complicated the rheological limitations. Achieving these prerequisites with less equipment in an environmentally sensitive area posed an interesting problem.

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