4D Seismics Improves Reservoir Management - A Case Study On Angsi
Authors: Mandal Dipak , Suwarlan Wahyudin , Abu Mansur Roslan , Ali Kartina , Masoudi Rahim 
Published: March 2015

Prudent reservoir management has ever been a challenge in water injection reservoirs. Monitoring this injection mechanism is essential to understand near term pressure response, evaluate pattern behaviour and take necessary measures to arrest decline and improve recovery. In Angsi water injection has commenced in reservoir I-X in early 2002. Strong production demand with premature watercuts in certain parts of the reservoir has posed a serious challenge to effective reservoir management. High water production in updip wells could not be explained by normal reservoir engineering tools. Tracer study initiated did not provide conclusive results. Seismic history consisted of 3D seismic in 1995 reprocessed in 2005 followed by new 3D data acquired in 2006. To understand the reservoir better, 4D seismic interpretation has been carried out recently. The results helped to explain the complex waterflow paths in the updip of the reservoir, which could not be explained by the depositional models or the reservoir engineering tools.

The seismic response change with time to match the saturation and pressure changes was carried out. The challenge due to data uncertainty from commingling and behind casing flow was overcome by critical reservoir engineering-petrophysics analysis. The workflow incorporated multidisciplinary inputs to minimize data quality concerns, establish 4D response algorithm for injected water to create a robust model.

The paper details how the 4D response algorithm was created and the results incorporated in the model to history match the updip high watercut. Based on the finding profile modifications have been carried out which helped to arrest decline and reduced unnecessary water recycling. Being successful, currently the response algorithm is being used to understand the complex water paths of another waterflood reservoir with the challenges of historical simultaneous injection at the gas cap.

Introduction

Industry experience has shown that managing waterflood in any reservoir has remained a challenging task. Understanding pattern behaviour, peripheral sweep, and preferential movement of the water paths have always been an uphill task. Premature water break throughs in updip wells, leading to banks of leftover bypassed oil poses a significant reservoir management issue. It not only lowers the recoveries of existing wells and thus the reserves associated with it but also leads to additional cost of lifting and processing recycled injected water.

In waterflood reservoirs, managers had thus always critically looked into well performance behaviours to understand the rise of water production. They have tried to map the water paths to get the best idea of the reservoir area getting swept. Mostly, the water movement between the wells remained controlled by the petrophysical and well test properties provided as per algorithms used in the reservoir model. Thus to match the well water production the reservoir engineer more often has to introduce local modifications. With time lapse 3D seismics, engineers have started looking into the understanding of these complex water paths by calibrating the seismic response with the dynamic water saturation.