Hydrocarbon-Mobility Steering for Optimum Placement of a Power Water Injector
Authors: Carpenter Chris 
Published: February 2015

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 164282, ’Hydrocarbon-Mobility Steering for Optimum Placement of a Power Water Injector Above Tar Mats - A Case Study From a Light-Oil Carbonate Reservoir in the Middle East,’ by Craig Saint and Thomas Glowig, Baker Hughes, and Ashis S.S. Swain, Nasser A. Al-Khaldi, Mohd. H. Al-Otaibi, Abdul Aziz Al Ghareeb, and Ahmad Bader Al Bader, Khafji Joint Operations, prepared for the 2013 SPE Middle East Oil and Gas Show and Exhibition, Manama, Bahrain, 10-13 March. The paper has not been peer reviewed.

Tar mats are encountered in many Middle East carbonate reservoirs. In cases in which tar acts as a flow barrier between water and light oil, water injectors work most efficiently when placed horizontally immediately above the tar mat to maintain pressure support during production. As this strategy is pursued, a real-time tar-detection method is needed while drilling. The low mobility caused by tar can be detected with a formation-pressure- while-drilling (FPWD) tester, while a nuclear-magnetic-resonance (NMR) tool provides oil-viscosity estimation.

Introduction

The Ratawi limestone reservoir was discovered in 1963 in the offshore Neutral- Zone Concession area between Saudi Arabia and Kuwait. The production of light sour crude oil commenced in 1966. After an increase in gas/oil ratio and a sharp drop in reservoir pressure occurred, dump-water injection was started in 1975. Until recently, peripheral dump-water injection was used.

Geologically, the reservoir is divided into three units, Units A, B, and C from top to base, respectively. Of these, Unit C is the main producing unit. The top of Unit C is considered as a possible sequence boundary/unconformity. Units A and B represent initial flooding over Unit C. The two units are composed of alternating porous limestone and less-porous or tight limestone, and show overall transgressive-system tract sequence.

The heavy-oil zone was recognized above the oil/water contact. Additional drillstem tests and investigation of the separation between shallow and deep resistivity indicated fieldwide distribution of the heavy-oil zone. Wireline NMR and pressure data indicated presence of a tar/asphaltene zone in the heavy-oil zone.

The operator’s strategy was to carry out peripheral water injection, placing the injectors above and as close as possible to the heavy-oil/tar zones, enabling better sweep and volumetric displacement of light oil toward potential producers. The reservoir characterization around the injector locations is critical for assessing carbonate heterogeneity, diagenetic uncertainty, asphaltene presence, and a transition zone of heavy-to-light oil. Assessment of risk-evaluating conventional- wirelinelog responses, seismic attributes, and nearby well dynamic information resulted in good prognosis of the area for deciding the entry point or landing point of the injectors and was crucial in well.placement.

Offshore operations did not allow evaluation of the tar/asphaltene zone in each well because of higher operating costs from drilling a pilot hole. In recent wells, the placement was achieved on the basis of mobility steering as close as possible to the heavy-oil/tar zone.