Abstract

Time-lapse cross-well electromagnetic (EM) surveys are used to monitor two types of fluid injection (Water Injection and Water Alternating Gas) in a giant field in the Middle East. Cross-well EM data will help optimize sweep efficiency, identify bypassed pay, and predict fluid-related issues such as water breakthrough by providing an image of the resistivity distribution between boreholes in time lapse. This paper explores the influence of a high quality background geologic model in constraining the interwell results and providing a higher resolution image of the ongoing flooding processes.

The classic EM inversion process determines a coarse (3 to 5 m resolution) resistivity distribution from a basic initial static reservoir model built from logs. This study refines the model by adding variable resolutions to encompass the small-scale heterogeneities common to carbonate reservoirs. Incorporating geological data derived from seismic attributes, core descriptions, and detailed log analyses into the static model helps optimize the EM inversion and increases the resolution of the resulting inverted model.

Introduction

A few years after the discovery of a giant complex carbonate reservoir in the Middle East, Giant Field A (Fig. 1), peripheral water flooding was successfully initiated to maintain pressure. Recently, it appears that reservoir complexity has led to uneven sweep. ADCO is currently testing pattern-based flooding technologies to improve sweep efficiencies at two pilot studies—the water injection (WI) and water alternating gas (WAG) pilots—to monitor the under-swept lower units of one of the main reservoirs in this field, which are not being swept with the peripheral flood.

Although pattern flooding leads to more efficient and faster recovery, some potential drawbacks include greater costs and higher local pressures, which could induce uneven flows. Detailed pattern flood modeling helped develop an optimum strategy for maximizing reserves and production, especially in the lower two oil bearing units of the reservoir (Bhatti et al. 2006). Consequently, water injection has been implemented for these lower units of the reservoir. A detailed multi-year and multi-measurement monitoring plan has been established to determine the performance of this pilot study, including deep reading technologies like cross-well electromagnetic surveys.

The WI pilot was designed to determine sweep efficiency in the targeted reservoir units while assessing the impact of injected fluids on low permeability subunits and monitoring pressure support due to pattern injection. Uneven sweep, bypassed oil, and residual oil saturation are secondary considerations of the WI pilot study. Production, injection, saturation, and pressure data will be used to calibrate the simulation model. The ultimate goal is to design an optimum field development scheme for the lower reservoir units in the southern part of the field (Bhatti et al. 2007).

Assessing pilot performance and fine-tuning the model's predictive capabilities requires proper surveillance, planning, and timely data gathering. To efficiently meet the pilot objectives while acquiring high-quality inter-well data, traditional methods were enhanced by the addition of more advanced (deep reading) methods. Selected well-based monitoring methods include well logs and pressure and flow data. Evaluation of a number of advanced geophysical methods led to the selection of the crosswell EM method for interwell saturation monitoring (Bhatti et al. 2007).

This paper shows the results of the first time lapse in the WI pilot, where decrease in resistivity due to 4 months of water injection in the lowermost units of the reservoir have been identified and interpreted with respect to the geological understanding of the pilot area.

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