Malaysia is a significant gas producer and LNG exporter within Asia-Pacific region. Many of the country's gas fields are offshore carbonate reservoirs. The exploitation of these reserves involves drilling horizontal wells for maximizing reservoir contact and hydrocarbon drainage. Many of these wells experience drilling mud damage. One of the challenges in stimulating long horizontal wells with open-hole completion is the placement of stimulation fluids for effective zonal coverage and generating wormholes to pass the damaged zone.
Placing gelled acid through coiled tubing has been the standard practice to clean up the wellbore. Due to the low pumping rate, stimulation results have been limited. A change was initiated aiming to have the acid pass the damaged zone and generate wormholes through effective diversions.
This paper describes the application of acid/diversion systems and pumping schedules to improve acid coverage. The selection of in-situ crosslinked and particulate diverters through laboratory testing is described as well as the implementation of a unique mixing system. Several case histories are presented illustrating the effectiveness of the fluid systems and mixing process in improving well productivity and job economics.
A stimulation campaign was executed in 2006 to utilize the systems selected in long open-hole horizontal carbonate gas wells. To maximize the results, the following have been practiced:
Clean up and test the wells after completion to get initial productivity.
Bullheading acid to increase pumping rate.
Use a mixing-on-the-fly unit in limited barge space.
Learn from previous jobs to improve subsequent ones by optimising pumping procedures.
Clean up and flow test right after stimulation to evaluate results.
7 wells were stimulated in the campaign. Significant gain has been achieved through progressively optimising pumping procedures. Indication of diversion was observed compared to previous practices. Stimulation cost is less than that of using coiled tubing.
In Malaysia under the Production Sharing Contract (PSC) with Petronas, Sarawak Shell Berhad together with its equity partners are under contract to develop and produce natural gas resources. Under the PSC framework gas produced will be supplied under a gas sales contract to the MLNG plants as shown in Figure 1.
Long horizontal wells are drilled where there are no compartmentalization or baffles in the carbonate reservoirs. In addition the wells are completed with large tubing size (7.5/8" tubing). The wells are cleaned up using a clean up package; stimulated with acid; further cleaned up before being tied into the production facilities. Tradiitionally coiled tubing has been used for the placement of acid.
Acid treatment volume used is typically 10 gal/ft, which is relatively low. The gelled hydrochloric acid (HCl) is mixed in batch tanks. Typically the average horizontal length is 2000 ft and the mixing of 20000 gal can take up to 24 hours due to the confined space on the drilling barges and the number of acid carboys available. Handling and mixing large volume of acid can be a logistical nightmare especially when there are more than 2 horizontal wells that require stimulation per campaign.