During the past four years Saudi Aramco has used short radius horizontal and multilateral drilling technology to increase oil recovery, productivity and injectivity of existing production and injection wells. This was achieved by(a) sidetracking oil wells with thin bypassed oil column and drilling shortradius horizontal holes at the top of the reservoir to control water coning and increase oil recovery, (b) drilling short radius horizontal holes across thin reservoirs and increase the wellbore flow area and well productivity, and (c)drilling multilateral short radius horizontal holes to increase the productivity and injectivity of existing oil and water injection wellscompleted in low permeability reservoirs. This paper presents well casehistories that describe the drilling and completion operations and compare the performances of the horizontal and conventional wells.
The Arab-D is the major oil producing reservoir in the Ghawar field in SaudiAramco. The reservoir is dolomitic limestone and has a net thickness of about 250 ft. The formation porosity and permeability are highest at the top of the reservoir and decrease gradually to ± 5% and few millidarcies at the base of the reservoir. The Arab-C reservoir which lies above the Arab-D is, in someareas of the field, a water-bearing and highly pressurized reservoir. The two reservoirs are separated by 100–170 ft of anhydrite cap rock and the thin postArab-D stringer which is oil-bearing in some areas of the field as shown inFig. 1. Most wells in the field are initially completed open-hole in the Arab-Dwith the 7" liner set at the top of the reservoir. The post Arab-D stringer isnot produced in wells completed in the Arab-D reservoir. The Arab-D reservoir pressure is maintained by the injection of water on the flanks of the field. Aswater injection continues, the water cut increases until the wells can nolonger flow on their own, leaving 25–35 ft of unswept oil column at the top of the reservoir.
One way to recover the unswept oil is to run and cement a 4-1/2" liner across the open hole and perforate across the unswept zone. The disadvantage of this method is the problem of water coning. As oil production from the unswept zone continues, the water cone rises around the wellbore and as a result the water production increases until the well dies before the oil in the unswept zone is recovered.
Applications of Short Radius Horizontal Drilling :
Short radius horizontal drilling technology was used to increase recovery of the unswept oil at the top of the Arab-D and to increase the potential of oil and water injection wells. Existing oil producers and water injection wells in the Ghawar field were reentered and short radius horizontal single and multilateral holes were drilled. This technique provided an attractive cost benefit compared to drilling new wells since the infrastructure is already inplace. Short radius horizontal completions were utilized in four major applications outlined below.
Recover Unswept Oil from the Top of the Arab-D Reservoir. Many oil producers in the Ghawar field ceased to flow because of high water cuts leaving 25-35 ft of unswept oil at the top of the reservoir. Reentering the wells and drilling short radius horizontal holes across the top of the reservoir proved to be the most economical and effective way of recovering the unswept oil.