Many horizontal/multilateral wells are drilled using water-based mud. In general, drilling fluids consist of xanthan, starch or polyanionic cellulose with bridging agents like sized calcium carbonate and salt particulates.During drilling and completion operations, it is essential to form an external filter cake to control fluid losses. However, effective filter cake removal during well completion is very difficult to achieve, especially in low permeability formations. Increasing use of long horizontal and multi-lateral wells with very low draw-down makes efficient cleaning and stimulation treatments even more challenging. Placement of corrosive acids to dissolve carbonate is problematic particularly in horizontal wells with extended reach. Polymer-breaking enzymes and oxidizers are effective at degrading polymer residues present in the wellbore, but will not attack carbonate or other acid-soluble materials. A new fluid addresses several key problems inherent in removal of drilling fluid filter cake, cleaning, and stimulation treatments of horizontal and multilateral wells. This fluid generates an organic acid (acetic or formic) in-situ, thus providing a very controlled acidising process for mud solids. The fluid can also incorporate enzymes to remove biopolymers. Several lab experiments were conducted to examine the compatibility of fluids with enzymes. The kinetics of acid producing reactions were examined in detail. A modified HPHT fluid loss cell was used to assess the effectiveness of the system in cleaning drill-in fluid damage. The surface tension of fresh and spent acid was measured as a function of temperature. Lab results indicated that the fluid is compatible with the enzyme system. Also, the fluid is non-corrosive and has minimum impact on well tubulars. Unlike conventional reactive acids such as HCl, the slow reaction kinetics ensure better distribution of the new fluid across long sections with minimal leak-off during pumping. The HPHT tests showed a significant improvement in the efficiency of removing filter cake. Surface tension measurements indicated that the spent acid solution had low surface tension, which will accelerate lifting these fluids from tight formations. Introduction Horizontal wells are widespread due to their high production potential, ability to access remote reserves and the lower number of wells required developing prospects.[1] Selection of an appropriate drill-in fluid (DIF) and clean-up procedure are critical in ensuring the productivity of openhole wells, particularly horizontal/multilateral wells. Effective cleanup of horizontal wells following drilling is required in order to ensure that the expected benefits are obtained and to maximize the Net Present Value (NPV) of the asset. The aim is to achieve uniform removal of the drill-in fluid filter cake from the entire completion interval.[2] Drill-in fluids are relatively clean compared to conventional mud systems and are designed to minimize the effects of drilling fluid damage when drilling through the pay zone. Drill-in fluids are typically formulated to deposit a high quality, relatively impermeable filter cake, which seals the wellbore and minimizes fluid leak off into the formation. In theory, such cakes may be removed by the produced fluids and from sufficient drawdown in the production direction.[3,4] In practice; cleanup may be uneven due to heterogeneous reservoir characteristics across the horizontal open hole. This can result in variable drawdown and/or variation in the extent of initial damage across the production interval. Typically, the part of the wellbore that has been exposed to the drill-in fluid for the longest period is the most damaged. If left in place, the filter cake can significantly reduce the production rate of the well, cause a poor production profile and reduce the efficiency of the completion. Production complications such as gas or water coning can also arise. Water-based DIFs comprised of carbonate particles and polymers are widely used in the industry. Calcium carbonate particles act as a weighting and bridging agent whereas polymers such as starch and xanthan provide appropriate rheological properties to the DIF. Such DIFs form filter cakes containing carbonate, polymers and rock fines arising from drilling.[5]
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