Lessons learnt and experiences gained in developing heavy viscous oil field using pattern waterflood. Case history of a field in South Oman
Authors: Gupta Vipin Prakash , Barrio Claudio Alberto , Al Mahrooqi Al Khattab Salim 
Published: November 2008

The paper describes the history of a successful field development of a complex heavy oil field in south Oman and illustrates the changing development philosophy dictated by the field behaviour.

The first field development plan was based on water flooding through horizontal wells in triplet (one horizontal injector flanked by two horizontal producers) patterns. The high injection rates needed to support the horizontal oil producers, induced reservoir fractures and consequently caused rapid water breakthrough at the oil producers. Subsequently, this development was put on hold due to the steep decline in oil rates.

An alternate field development plan based on an inverted 5 spot pattern water flood was therefore designed adopting injection strategies designed to restrict fracture size over long periods of time with increasing rates in a controlled manner. Other subsurface complexities such as reservoir compartmentalization, erosion of producing formation and sand continuity, were addressed using a vertical well pattern approach which ensured that each producer well would be part of 4 patterns and thereby increasing the chances of getting the benefit of injection and overcoming subsurface complexities.

The results of early development were incorporated to generate simulation models that formed the basis for the injection strategy, and get better understanding of the reservoir injectivity and propagation of induced fractures due to water injection.

The successful implementation of the inverted 5 spot pattern waterflood resulted in arresting field decline from 17% p.a. to 10% p.a. Other indications of positive results were represented by the reservoir pressure increase from 1500 kPa to 4000 kPa seen since 2005 when the first pattern was put in place.

A phased approach that considers subsurface spatial variations and a time plan that takes into account the development learnings and change implementation is the most effective approach for an optimised field development plan.

Introduction

The paper tracks the development history of an oil reservoir located in south Oman containing heavy and viscous crude (22 API, 90CP), on production since 1980. The initial development was based on primary recovery (depletion/solution gas drive mechanism) using vertical wells.

Continued production without adequate support led to fall in reservoir pressure and a plan to develop the field through line drive injection using triplets (a horizontal injector flanked by two horizontal producers) was put in place aiming at a ultimate recovery of 24%. Initially encouraging, the response became increasingly negative with steep fall in production and rising water production. It was apparent that rapid breakthrough through possible fault fractures has resulted in short circuiting. A period of investigation/study/ data collection followed using waterflood pilots to understand the involved processes and come up with a development strategy. The latest FDP (Field development Plan) envisages development through pattern flood using vertical injectors and producers. The program running for two years has been able to demonstrate successful waerflood in a adverse mobility scenario on a sustained basis.