Abstract

The Manusela Formation in the Oseil Field is a highly productive fractured carbonate reservoir. The field has a reservoir pressure of 2,522 psi at 5,600 ft-TVDSS (equivalent mud weight of approximately 7 ppg) and contains 15 to 22 degree API gravity oil with minimal associated gas. The surface location has an elevation of approximately 1,300 ft, and therefore wells require artificial lift to produce.

The discovery well, Oseil-1, was drilled conventionally in 1993 and tested with an Electrical Submersible Pump (ESP), but suffered significantly from lost circulation and formation damage. Reservoir characterisation was difficult; cores recovered were swept clean of hydrocarbons by the circulation losses, and the oil water contact was impossible to determine with any degree of accuracy. The well did prove up significant reserves but further wells were needed to better characterise the formation.

Two appraisal wells, Oseil-2 and Oseil-4, were subsequently drilled and cored in 1998 using a nitrified diesel/crude oil drilling fluid. Electronic pressure recorders were run above the bit to obtain formation pressure information and productivity estimates while drilling. A Progressive Cavity Pump (PCP) was used as a cost effective artificial lift method for the testing program.

Although not on an incremental "foot-by-foot" basis, productivity was determined section by section dependent on the drilling program. Oseil-4 proved easier in this regard because of the relative lack of complications with the additional free formation gas associated with drilling Oseil-2. In addition the underbalanced drilling technique resulted in better core quality.

The PCP was used in conjunction with a pressure operated downhole shut in valve for testing. Its ability to pump high viscosity fluids with minimal surface equipment made it the most suitable tool for this remote region of Indonesia. An unexpected gas cap encountered in Oseil-2 caused several operational problems with the PCP, including excessive spin, back spin and a drop in pump efficiency. Procedural changes enabled these problems to be overcome in many cases.

Well productivity in excess of 600 bpd/psi resulted in minimal drawdown while testing. Offset well interference data was affected by earth tides. With an amplitude of the order 0.2 psi, the tide was a significant proportion of the pressure transient of between 7 and 8 psi. Corrections were performed using a dedicated earth tide generation program.