Abstract

In this paper we describe a case history of a raging battle against scale deposition in the production system of Tinggi oil field in offshore Terengganu, Malaysia. This field experienced a sharp production decline from 1995 to 1997 at a rate of 26% per year. Scale deposition in various parts of the production system together with sand and emulsion caused widespread equipment/instruments failure, valves/chokes malfunction and most importantly severe piping restrictions at various points of the entire production system, surface and subsurface. A facilities capacity review study was conducted in order to differentiate between real facilities bottlenecks and scaled up restrictions. Pin pointing all the scaled up bottlenecks was quite a challenge. Diagnosis methods included visual inspection, pressure drop comparison, temperature sensing and gamma ray scanning. All scaled up points were successfully detected and cleaned up. As a result of this work together with sand, emulsion control and other production optimization efforts, oil production started to pick up by mid 1998. Production increased by 64% within a year with a total gain of 3.7 MSTB/d.

A comprehensive scale management plan was put together, both short term and long term. Routine scale cleanup, identification of scale-prone source reservoir and its management, scale inhibitor chemical injection program, routine stroking of control valves, scale buildup monitoring program of surface flowlines are some of the short term measures taken. Amongst long term measures, a study of effectiveness of Magnetic Fluid Conditioners (MFC) has been initiated. One commercial magnetic tool has been tried so far, with no benefit observed. While long term study continues, short term measures have brought the situation completely under control.

Introduction

Tinggi field is part of the PM-9 block, located in the south-eastern part of the Malay Basin, approximately 280 kilometers offshore east of Kerteh, Terengganu, Malaysia (Fig.1). This field was developed during August 1982 to March 1984. Tinggi reservoirs (J & K) produce under a combination of natural water drive and gas re-injection (in upper J only) at the crest of the small gas cap. About 92% of their UR (123.0 MMSTB) has been depleted with current WC of 85% and GOR 900 scf/STB.

Scale Deposition Problem

Tinggi production peaked at around 40 MSTB/d in 1989 (Fig.2). Then as the field water cut started to increase, signs of scale deposition on surface facilities piping started to become visible. Sand production and emulsion problem also started to surface. Gradually several wellhead flowlines and downhole tubing started to experience scale deposition resulting in flow restrictions and well accessibility problems. Wellservices work started to become more and more difficult as accessing some of the wells required tubing clearance using mechanical scratching and/or chemical (acid) descaling. Well surveillance and maintenance became more expensive. Essential surveillance work often could not be performed due to unpredictable scaling restrictions. Well remedial/enhancement work became difficult, expensive and at times very frustrating. Number of idle wells started to rise as the operations people were not being able to cope with the amount of challenges posed by the wells and process facilities as the scale deposition, sand production and emulsion problem started to cause widespread frequent equipment & instrument malfunction. Consequently, compressor trip, process upsets etc became the normal order of the day. Production declined sharply at a rate of about 26% per year over '94 to '97. In early '98 it was producing an average 5.8 MSTB/d of oil with a water cut of about 90%. That decline trend together with the operational challenges (sand, scale and emulsion) threatened the economic viability of Tinggi operations.

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