The development of light-oil reservoirs in Kuwait has become increasingly more important for maintaining the quality of exported crude. This is due to the fact that producing light-oil reservoirs has proven to be not only a necessary link for maintaining the oil production from this area but also for increasing it. As a result, efficient testing of the light-oil reservoirs has become paramount in importance for overall well and field development.
The reservoirs in Kuwait are low permeability, high pressure/high temperature (HP/HT) and sour. In earlier wells, the strategy had been to perforate these formations balanced or slightly overbalanced, in mud, with through-tubing guns. Results from testing in several wells indicated that if they could be perforated under-balanced, the formations would yield better results since this method would allow better penetration and perforation cleanup. A number of reservoirs are stacked horizontally and range from typically conventional to fractured limestone.
In view of the corrosive nature of the fluids present, one of the primary efforts in the testing of these wells had to be directed toward keeping the number of wireline and coiled-tubing operations to a minimum without compromising the testing objectives. Since it was necessary to test multiple objects in the exploratory wells individually, special effort was focused on determining methods that could effect a reduction in the testing period for each object. By reducing individual testing times, Kuwait Oil Company felt that the overall savings would be significant.
Several areas were identified that would require special consideration. This paper will discuss these areas (listed below) and how the challenges they presented were addressed:Modifications to retrievable completion test (RCT) tools and tubing-conveyed perforating (TCP) equipment that would be required to test these zones efficiently.The challenges associated with designing and carrying out the tests.Ongoing modifications in the testing methodology to overcome the testing challenges.The operation of the test-string, annulus-pressure-responsive components in the heavy oil-based mud and the difficult wellbore conditions.How the problems of shaped charge performance in the naturally fractured formations with unusually high compressive strength and very low matrix permeability and porosity were resolved.
The modifications to tools and methods allowed the goals of the operator and service provider to be met.
Background
Fig. 1 shows a typical completion schematic for the formations intercepted in the deep Jurassic layers of west Kuwait. Monobore completions are normally used. The reservoir evaluation technique followed by the Kuwait Oil Company in the wells of west Kuwait follows:Drill and complete well in the zone of interestMobilize and rig-up wireline perforation unit and production testing spreadPerforate the well using through-tubing guns with mud capsFlowtest the well through the production testing unit on location.
While the above procedures were satisfactory, it was felt that that better well performance and completion efficiency could be achieved by a step change in the formation evaluating technique. Other options were reviewed.1,2,3
The first change planned was to test the well using a retrievable completion test (RCT) string. The reasons for making this change were due to the testing methodology adopted and the changes made to the procedural operations. These changes follow:Since the wells were deep with a telescopic casing design, and the reservoir was sour, the decision was made to test using completion tubing instead of the drill string.A Christmas tree was to be used as opposed to a surface test tree with testing performed through the blowout preventers (BOPs) as in a regular test. Safety was the primary driver for using a Christmas tree as the well had H2S and high pressures.
