Water-based fluids have been used to drill various wells in Saudi Aramco fields. A typical water-based mud consists of xanthan, starch or cellulose polymers, plus bridging agents like sized calcium carbonate or salt particles. The polymers in the drill-in fluids are needed to build the required viscosity to transport cuttings and form a filter cake around the wellbore to reduce the fluid loss. Some of these polymers invade the critical near wellbore area and may cause formation damage.
One of the treated wells was the first multi-lateral ever drilled in a deep sandstone gas reservoir in Saudi Arabia. The dual-lateral well was completed with a pre-drilled liner completion. Mineral and organic acids can be used to remove the filer cake, but they cannot be applied in horizontal and multilateral wells without using an effective diversion technique. In addition, the sandstone formation in this case is acid-sensitive. As a result, a filter cake cleanup process was performed separately in each lateral, using a completion brine wash followed by a specific enzyme treatment.
Several lab studies were conducted using field mud samples to determine the cleaning efficiency of enzymes used to remove damage which occurred while drilling the target zone. For the first time, an enzyme solution was spotted in one lateral and left in the hole for more than four months, until the second lateral was drilled and the well was ready for testing.
Various tests were conducted on well flowback samples after the enzyme treatments. These tests included: chemical analysis of the aqueous phase and full analysis of the produced solids using acid solubility, sieve analysis, XRD, XRF, and SEM.In addition, the carbohydrates content in the aqueous phase was measured using the anthrone method.
A significant improvement in gas production from this well has been realized through the application of the enzyme treatment for the removal of starch and XC polymers used in drilling operations. This paper will discuss in detail laboratory work that was conducted to assess this field treatment.
Drilling of horizontal/multilateral wells is increasingly widespread, primarily because of the enhanced productivity they provide from many types of reservoirs. These wells often use uncemented predrilled liner completions. Perforated liners are typically used in small-diameter wellbores or in sidetracks within the reservoir where there is no need for the liner to be cemented in place. Because these wells are not perforated, mud damage cannot be bypassed. Therefore, to maximize well productivity, preventing or removing mud damage to the formation before bringing the well onto production is important.
Long horizontal/multilateral wells present a major challenge in wellbore cleanup operations. This is because the length of the target zone complicates the process of uniform filter cake removal and further dictates the need for a very effective clean-up system.
Water-based fluids are increasingly used in the field. However, they create mud filter cake which can cause formation damage. The filter cake comprises solids, such as calcium carbonate particles, formation fines, and polymers such as xanthan and starch. A lower fluid loss is desirable in order to minimize fluid invasion into the formation. Fluid invasion can cause formation damage and unstable bore hole conditions. A high fluid loss results in a thicker filter cake and can cause problems with drill-pipe sticking.[3,4]
Fluid loss reducers include starch and polyanioinc cellulose. These polymers can limit the loss of drilling fluid to under-pressurized or high-permeability formations. Starches have thermal stability to nearly 250°F.In many of the drilling fluid formulations, the weight ratio of starch to XC-polymer is more than unity, and thus removal of starch alone from the filter cake can significantly reduce the flow intiation pressure and permeability impairment.