Lost Circulation Challenges: Drilling Thick Carbonate Gas Reservoir, Natuna D-Alpha Block

Abstract

Abstract The L-Structure in the Natuna D-Alpha Block of the East Natuna Basin is located in the Natuna Sea in Indonesian waters, northwest of Kalimantan. The structure is a large structural stratigraphic hydrocarbon trap and is unique in areal extent and reservoir thickness among the various carbonate traps which are present in many southeast Asian basins. The porous reef complex (Terumbu formation) has a fairly homogeneous gas column 5,200 feet thick with C02 content averaging 72%. The purpose of this report is to summarize the exploration drilling problems in the reservoir relating to severe lost circulation occurrences and to explain the steps which have been taken to alleviate these problems. The thick Terumbu reservoir is a fractured, vuggy limestone. Furthermore, the long gas column in the reservoir presents a pore pressure profile which ranges from abnormally high in the top, down to a fresh water gradient in the bottom. The above factors compound the task of dealing with lost circulation and lead to additional drilling problems. Much progress has been made in reducing the severity of lost circulation and lessening its effects on drilling operations. Careful casing seat selection, improved drilling practices, strict control of drilling mud properties and use of lost circulation material in the mud while drilling have been major contributors to successful penetration of the entire reservoir. Introduction The Natuna D-Alpha Block, Natuna Sea, offshore Indonesia (Figure 1) is being explored by Esso Exploration and Production Natuna Inc. under a Production Sharing Contract with Pertamina, the Indonesian government oil company. The very large gas-bearing L-Structure dominates the central part of the block (Figure 2). The discovery well, AL-IX, drilled by AGIP in 1973, encountered about 5200 feet of gas-bearing carbonate rocks (Figure 3) in the Terumbu formation. ESSO drilled and evaluated four more exploration wells on the structure between November 1980 and September 1983 using floating rigs in 450 to 475 feet of water. The Terumbu formation has a fairly homogeneous gas column with C02 content averaging 72% and H2S content averaging 0.6%. The formation is sometimes vuggy and sometimes fractured. These characteristics combine to make it highly susceptible to severe lost circulation and other related drilling problems- Lost circulation is the most difficult challenge in Natuna. Nearly all the other problems which have been encountered, such as kicks, stuck pipe and sidetracks, have been direct results of lost circulation. Considerable progress has been made over the course of the exploration drilling campaign learning to cope with the lost circulation problem. LOST CIRCULATION OCCURRENCES AND SEVERITY Lost circulation has been encountered in all sections of the Terumbu formation (Figure 4). There does not seem to be any correlation between lost circulation occurrence, depth of penetration into the formation or mud weight overbalance. Lost circulation has occurred near the top of the formation with 0.9 ppg overbalance, and in the bottom of the formation with as much as 2.9 ppg overbalance. The severity of these losses are shown in Table I. Loss rates as high as 600 barrels per hour were observed. P. 947^