Abstract

Exploitation of ultra-thin oil columns under gas cap and water support is challenging since both gas and water coning can seriously curtail oil recovery. Horizontal wells have shown to alleviate some of this problem as they allow for less drawdown and hence reduce coning for better recovery efficiency. Although the industry has shown horizontal drilling cost continues to be lowered, economics is still a major issue in exploiting ultra-thin oil bands using horizontal completion.

This paper presents a case study showing lessons learned from managing reservoirs with ultra-thin oil bands with less than 20 ft and sandwiched between gas cap and bottom/edge aquifer. Actual data from a field characterized by stacked pays of fluvial and deltaic channel sands in the Mahakam Delta complex are demonstrated. Subject to long-term production, the originally thick oil columns in these reservoirs have now become thinner, yet carry significant reserves to be prudently further developed. Issues involved in reservoir management such as well surveillance, well planning and prognosis, operating while drilling and producing, and long-term development plans are discussed. Reservoir modeling is shown to be the guide not only for well planning but also for well operations as it is used to select well type and completion strategy, optimize well production and prepare for continuous annual operating plan.

The vast well performance database provided from the paper shows horizontal drilling continues to be the main vehicle to develop these ultra-thin oil bands. Care however needs to be taken to scrutinize the economics inherent in selecting this type of completion, as other alternative methods also deserve to be considered.

Introduction

Mature oil reservoirs with gas-cap and water support have become the prime targets for horizontal drilling by many operators in the last fifteen years. Operating in East Kalimantan, Unocal Indonesia Co. (UIC) has extensively used horizontal applications in developing their mature fields by completing more than 100 wells since 1996. Although horizontal wells have proven their performance to develop thin oil bands by providing better oil recovery efficiency, the remaining, ultra-thin oil pays (e.g. less than 20 ft) in these reservoirs that need to be further developed, pose higher risk with horizontal drilling. As expected, recovery naturally becomes less with thinner oil columns, horizontal drilling becomes more costly as it requires more accurate geological interpretation and sophisticated equipment that would have negative effect on the project NPV's. To alleviate risk and improve economics, feasible options to be considered are (i) short- to medium-radius horizontal completion for single zones, or (ii) conventional completion penetrating several stacked pays.

The paper presents a case study to demonstrate lessons learned from exploiting ultra-thin oil bands under 20 ft in the Attaka field and its plan to continue developing these remaining oil columns using horizontal technology. The paper first describes the Attaka field and discusses its recent horizontal drilling program and results based on performance data from 13 horizontal wells drilled in various ultra-thin sands. Reservoir management issues involved in drilling and operating these wells are briefly reviewed. To support future plan of development, a reservoir model based on field data is used as a basis to examine various completion strategies. Effects of reservoir and well conditions on recovery are noted from results of many sensitivity runs that examine variations in well length, well placement standoff to fluid contacts, tubing size, initial rate, and effect of gas lift. Reserves and economic evaluations (NPV) are then used as guidelines for the future development plan of the remaining ultra-thin oil columns in the field.

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