Near-wellbore formation damage is expected during the drilling operations. Minimization, prevention, and removal of near-wellbore damage are essential to maximize well productivity. One of the major sources of skin damage is the residual filter cake developed by reservoir drill-in fluid (DIF). Therefore, an efficient filter cake cleanup method should be considered to enhance well productivity. Oil producers in sandstone reservoirs are being drilled with invert emulsion DIF during oil reservoir drilling. Standard completion of oil wells is with stand-alone screens. Although screens stabilize the wellbore and address sand control issues, it can also act as a trap for filter cake, resulting in high drawdown pressure. An in-situ acid-precursor technology, which generates organic acid, was proposed to be used. The prime advantage of this technology, especially in long horizontal wells, is the uniform distribution of acid during precursor to remove oil-based mud (OBM) filter cake by a single-stage. A special surfactant blend was incorporated in the cleanup fluid to alter the wettablity of the oily filter cake, and therefore, facilitate the reactivity of the produced acid with calcium carbonate particles. Detailed lab studies were conducted to evaluate the efficiency of the in-situ acid generator technology. The lab studies include various tests to evaluate filter cake removal efficiency, return permeability on real core samples, compatibility with formation fluids, and strength of dissolving calcium carbonate particles. Based on lab results, it was recommended to spot the single-stage filter cake cleanup fluid as a treatment for an oil well. This paper will discuss in detail the laboratory work that was conducted to evaluate the filter cake cleanup fluid performance. Introduction Minimization and removal of formation damage is an ultimate goal in drilling and completion designs to reach the target production rate. Field practices and numerous case studies have shown that near-wellbore damage is mainly caused by the drill-in fluid (DIF), which may result in major production restriction. The damage is caused by a form of external damage, such as mud cake. Invasion of mud solids and filtrate is a major source of the near wellbore damage (Almond et al., 1995; Leschi et al., 2006). The conventional near-wellbore damage removal or filter cake cleanup methods by chemical means are enzymes, chelating agents, reactive mineral acids, oxidizers, or a combination of these chemicals (Davis et al., 2004; Davidson, E. et al., 2006; Al-Otaibi et al., 2004). When it comes to removing an oil-based mud (OBM) filter cake, prior stages should be considered to these conventional treatments. The wettability of the oily filter cake particles must be altered from oil-wet to water-wet. This is usually done through multi-stages and consumption of large volumes of solvents, mutual solvents, and surfactants mixed in a carrying fluid. Eventually, for an operator, completing a well drilled with an OBM will be time-consuming and expensive using the mentioned conventional treatments.

Formation damage minimization and removal are important factors in enhancing field productivity and achieving targeted production rate. While formation damage is anticipated during drilling, an effective wellbore cleanup fluid becomes essential to remove mud damage and enhance well productivity. Filter cake layer created by drilling fluid can impair production considerably and should be removed prior to production using an effective treatment. Filter cake can be removed using a chemically designed filter cake removal treatment. The design of the treatment requires considerations of mud type and reservoir conditions. Also, it is important to consider the compatibility aspect of the treatment fluid with formation fluids and drill-in fluid base oil. Incompatible fluids result in precipitation which leads to further formation damage. In Saudi Arabia, many sandstone reservoirs are drilled with invert emulsion drill-in fluid to avoid the risk of clays swelling, which can lead to hole sloughing and formation damage. However, production impairment due to mud damage effect was observed in some horizontal wells drilled with invert emulsion drill-in fluid and completed with stand-alone screens as a result of improper mud managemet. After being put on production, their production rates have been reduced significantly with time. The decrease in production was related to screen blockage and mud cake impairment. A newly developed single-stage treatment was spotted and soaked to remove oil based mud cake and screen blockage. A significant sustained improvement in production was noticed and flowback solids indicated removal of screen blockage and mud cake. Lab studies were conducted at reservoir simulated conditions to evaluate the effectiveness of the treatment. The study covered testing the treatment filter cake removal efficiency, return permeability tests with reservoir core samples, and compatibility tests with formation fluids and drill-in fluid base oil. This paper will discuss in detail the laboratory work conducted and the successful field treatment performed.

Reactive mud cake breaker fluids in long open hole horizontal wells located across high permeability sandstone reservoirs has had limited success because they often induce massive fluid losses. The fluid losses are controlled with special pills, polymers and brine or water, causing well impairment that is difficult to remove when oil-based mud (OBM) drill-in fluids (DIFs) are used. This situation has resulted in the drive for an alternative cleanup fluid system that is focused on preventing excessive fluid leak off, maximizing the OBM displacement efficiency and allowing partial dispersion of the mud cake for ease of its removal during initial well production. The two-stage spacer application is composed of a nonreactive fluid system, which includes a viscous pill with nonionic surfactants, gel pill, completion brine and a solvent.Extensive laboratory evaluation was conducted at simulated reservoir conditions to evaluate the effectiveness of the OBM displacement fluid system. The study included dynamic high-pressure/high temperature (HP/HT) filter press tests and coreflood tests in addition to wettability alteration, interfacial tension and fluid compatibility tests.The spacer fluid parameters were optimized based on wellbore fluid hydraulic simulation and laboratory test results, which indicated minimal fluid leak off and a low risk of emulsion formation damage. Three well trials were conducted in a major offshore field sandstone reservoir drilled with OBM. All three trial wells (one single and two dual laterals), which were treated, have demonstrated improvement in production performance. This paper will discuss in detail the spacer fluids optimization process, laboratory work conducted and the successful field treatments performed.

No abstract