The onshore Dukhan oil field of Qatar Petroleum has been under production since the 1940's. Water is injected to maintain reservoir pressure, and a reservoir monitoring program has been established to monitor movements of oil-water (OWC) and gas-oil contacts (GOC) in the different reservoir zones. Historically, Pulsed Neutron Capture (PNC) logging (Sigma) was carried out for contacts monitoring and time-lapse saturation monitoring. In September 2007 Cased Hole Resistivity (CHR) was introduced for application in the Arab C reservoir.
CHR was chosen to overcome the recognized issue of Sigma logging in an environment of low contrast between oil and water response as a result of mixing fresh injection and salty formation water. It also has the advantage of a much greater depth-ofinvestigation compared with a Sigma measurement, which potentially can suffer from wellbore fluid re-invasion through existing perforations, thus adversely affecting the saturation evaluation. The slim version of the CHR tool used in Dukhan allows logging through existing small-size tubing, without the need to pull the completion string.
Proper candidate selection and quality control is essential for ensuring success. Results up to now have been very positive with identification of the oil-water contacts, enabling Qatar Petroleum to re-define the position of the OWC and monitor encroachment of the injection water, critical for the planning of new production wells. Previously, dedicated pilot holes had to be drilled in areas of larger uncertainty of the OWC. Most of the new wells in Dukhan Field Arab C reservoirs are now planned using OWC information obtained from CHR.
Cased hole resistivity data have until now not been used for saturation calculations, as the formation resistivity, especially in the lower porosity section, is close to the upper resistivity limit of the CHR tool. Projected uncertainties may be too large to determine saturations with the desired accuracy.
The successful application of the CHR helped Qatar Petroleum to overcome a limitation of Sigma logging in the Dukhan Arab C reservoir. It allowed accurate determination of the present-day OWC and encroachment of injection water. New horizontal producers were planned and drilled without the need for designated pilot holes.
The Dukhan oil field lies in the west of the Qatar Peninsula forming a long and narrow anticline about 70 km long and 8 km wide in N-S direction. The main producing intervals are the C and D reservoirs of the Upper Jurassic Arab formation, the Middle Jurassic Uwainat formation, and the Permian Khuff formation. Arab-C, Arab-D and Uwainat formations are oil reservoirs, the Khuff is a non-associated dry gas reservoir.
The Arab-C formation, subject of this paper, is a predominately limestone sequence intercalated with smaller dolomite layers, separated from other Arab reservoir zones above and below by tight anhydrite sections. The average thickness is 85 ft. The average reservoir porosity is 15% to 20%. The average permeability of the reservoir is approximately 30 mD but this varies widely based on reservoir layer.
Water injection is under way to support the reservoir pressure, and a reservoir monitoring program has been established to monitor movements of oil-water (OWC). Precise contact monitoring is essential for planning of remedial work and drilling of new horizontal production wells.