Smart-Horizontal-Well Drilling and Completion for Thin-Oil-Rim Reservoirs in Malaysia
Authors: Carpenter Chris 
Published: March 2016

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 17753, “Smart Horizontal-Well Drilling and Completion for Effective Development of Thin-Oil-Rim Reservoirs in Malaysia,” by Keng Seng Chan, Rahim Masoudi, Hooman Karkooti, Ridzuan Shaedin, and Mohamad B. Othman, Petronas, prepared for the 2014 International Petroleum Technology Conference, Kuala Lumpur, 10–12 December. The paper has not been peer reviewed.

For thin-oil-rim reservoirs, well placement, type and path, and well-completion methods, should be evaluated with close integration of key reservoir- and production-engineering considerations. This process involves maximizing reservoir-fluid contact and drainage, optimizing well productivity, and optimizing the production profile along the wellbore. Field-implementation cases in Malaysia have demonstrated that this integrated approach to horizontal wells can significantly minimize well count, enhance well performance, and improve ultimate recovery per well in thin-oil-rim reservoirs with varying reservoir complexities and uncertainties.

Introduction

Development of oil-rim reservoirs in Malaysia has improved progressively through a series of reservoir-engineering studies and successful field implementations. Although typical oil-rim reservoirs are characteristically wedged between a gas cap and an underlying aquifer, they can be structurally complicated by faults and flow boundaries, featuring various dips and saddles. Oil-rim reservoirs of various sand thickness can also be stacked and compartmentalized. Aquifer support can vary from one reservoir sector to another, resulting in uneven pressure depletion and fluid-contact change. Fig. 1 shows such a vast oil-rim reservoir in peninsular Malaysia.

Early oil-rim-reservoir field-development strategy consisted mainly of drilling a large number of vertical or low-angle wells. Wells are completed with multiple strings in order to produce separately from various different but stacked sands with significantly large pressure differences. Well life can be short, producing at high water cut and high gas/ oil ratio (GOR) because of the wells having partial penetration at high pressure drawdown. In such a well-development strategy, idle-well counts increase dramatically. Currently, active wells in some key oil-rim reservoirs are far less than 50% of the inventory. Field oil recovery remains low even after 25 years of continuous production.

Force Balance in Thin-Oil-Rim Reservoirs

There are several drive mechanisms dominant in oil-rim reservoirs—namely, gas-cap expansion, drive, viscous withdrawal, and gravity balance, in addition to solution-gas-expansion drive. Producing-gas reinjection; controlled pressure drawdown at wells in different reservoir sectors on the basis of varying aquifer strength and sand-layer dip angle; and the placement, spacing, and completion of the wells could all affect the balance of these drives. The art of this balance is to keep the oil rim in place, avoiding pronounced migration. A subtle balance of forces can keep the oil rim in place for many years with minimum pressure decline and more-even fluid withdrawal across the reservoir (Fig. 2).

Even though oil-rim reservoirs could have a huge underlying aquifer, the bottomwater support may not be adequate in some oil-rim-reservoir sectors. Some oil-rim reservoirs in Malaysia have observed a significant pressure decline. Water and gas injection should also be implemented, if not for the whole field then selectively in some regions, in order to preserve force balance, minimize oil migration, and improve oil recovery. Injection strategies and methods include periphery or fencing, updip water injection, and downdip gas injection.