ABSTRACT:

Petroleum Development Oman (PDO) has drilled 350 horizontal wells in the past 8 years in 33 different oil and gas fields. Since the first wells were drilled the technology and its applications have evolved considerably. The paper describes that rapid evolution Wing four fields as examples. There has been a diversification of well designs as we have learnt how to tailor horizontal drilling most effectively to different situations. In many cases wells can be drilled faster and cheaper than 5 years ago, but there are also examples where more elaborate designs have been applied. The geological targeting and evaluation of the wells has also improved. Further evolution is planned with the next step likely to be the wider we of multiwell bore horizontals.

Introduction

PDO produces 730 000 bbl/d (119 000 m3/d) of crude oil from some 70 fields of which 33% comes from horizontal wells. Horizontal drilling was started in 1986 with three short radius wells drilled in chalky oil reservoirs prone to gas or water coning and low production rates. Results were not sufficiently encouraging to lead to further activity in the short term. Horizontal drilling technology evolved rapidly and in 1990 PDO embarked on a more ambitious programme (Ref. 1). The results this time were so impressive that the trial was extended and has led to almost continuous horizontal drilling (80% of all wells drilled in 1994). So far 330 horizontal wells have been drilled of which 300 are operational producers and 20 are injectors (Figs. 1 and 2). Some 100 horizontal wells per year are expected to be drilled for the foreseeable future. These wells exploit both clastic and carbonate reservoirs from Precambrian to Cretaceous age, thin and thick oil columns, light and heavy oil, and a wide range of reservoir quality. Reservoir depth varies from 300 to 3000 m with up to 1900 m of horizontal section. Recovery processes include bottom and edge water drive, solution gas drive, gas-oil gravity drainage and waterflood (Figs. 3 and 4).

Horizontal drilling in PDO has shown significant benefits in increased production rates, reduced costs and increased oil recovery from existing producing fields and previously uncommercial oil accumulations. Well costs have been reduced by optimising the well design and drilling operations. The most significant cost saving has been due to a reduction in hole size and casing scheme. These improvements have been made without compromIsing well integrity or inItial production rate.

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