Carbonate Drilling with Mud Loss Problems in Offshore Sarawak
Authors: Taib Mohd. Anuar 
Published: December 2007

Abstract
This paper describes the problems and recently developed preventive and remedial measures to counter recurring mud losses when drilling wells in carbonate reservoirs offshore Sarawak. The frequency with which total loss of drilling fluid occurs varies over the fields and averages around once every six wells drilled. In some fields, however, this rate is as high as once every other well. The resulting well control problems, associated hazards and recovery costs justified preventive and remedial measures to be developed prior to drilling the recent M-field development wells in similar carbonate reservoirs.
Due to the existence of large voids in the carbonate reservoirs, upon hitting these voids, the annular overbalance is lost and consequently, gas percolates quickly up the annulus. Due to the inability to plug off the loss zone quickly enough as well as to sustain adequate annular fillup rates to prevent gas percolation, the wells have to be closed-in. In many cases, casinghead pressure reached alarming values while curing losses and more than once the drillstring became plugged and/or stuck.
This paper discusses various applied techniques that have been used to ensure wells could reach their objectives. Apart from several conventional options, drilling with floating mud cap and the use of fibre cement are introduced. These have significantly improved performance during the recent M-field development drilling campaign and is clearly demonstrated by comparing current performance with historical performance data.

Introduction
Sarawak Shell Berhad have since the late sixties drilled 103 wells in carbonate structures offshore Sarawak. To date, five gas development projects comprising of 31 wells have been drilled and completed in carbonate reservoirs with the M3 and M1 fields being the latest development projects. Potential mud losses in gas-bearing carbonate structures have been the issue attracting most attention in planning and executing the drilling programmes. This paper describes the problems, preventive and remedial measures to counter recurring mud losses when drilling these carbonate wells.
We have classified losses into four categories, namely, seepage, moderate, severe and total losses. Historically, of the 103 wells drilled, 45 wells have experienced mud losses in various degrees. The frequency with which total losses have occurred is once every six wells drilled. In the M1 field, however, the frequency is as high as once every other well. Figure 1 illustrates the categories and frequency of mud losses in carbonate structure in Offshore Sarawak.
Typically, the carbonate reservoirs are homogeneous, with gas column in excess of 100 ft and preceded by a thick column of shale. The formation pressure is usually above hydrostatic with reservoir content of either gas/water or some gas/oil/water sequence. The porosity of the fields ranges from 23 to 33 percent with permeability from 200 to 600 milliDarcy. Both sweet and sour gas reservoirs have been drilled with H2S content ranging from 0 to 1000 ppm. Among the gas development projects, the M3 field has the highest H2S content at 160 ppm.

Problem definitions
The majority of losses occurred in the lower part of the reservoir, hence below the top of the gas column. Losses occurred due to the existence of large voids and fractures in the carbonate reservoirs. These voids are usually localised and there are limited means to predict the existence of these voids.
Typically, the 9 5/8" production casing is set 200 to 300 ft above the top of carbonate.
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