Summary Successful completion of openhole horizontal wells requires removal of potential impairment from the near wellbore and formation face. Impairment can reduce well productivity and lead to failure of sand control screens due to plugging and erosion. Shell has adopted the practice in the Gulf of Mexico of employing drill-in fluids with acid-soluble bridging and weighting agents for all openhole drilling and extensive displacement and acid cleanup procedures during horizontal well completions. Drill-in fluid and filtercakes are removed with specially formulated sweeps and by attainment of sufficient velocity to achieve effective displacement. Filtercakes are removed with specially formulated washes applied using washcups or coil tubing to ensure that the cleanup formulations contact the filtercake materials. Drill-in fluids and cleanup systems must be specially formulated and evaluated for specific applications. We have adopted a suite of laboratory screening tests to evaluate formation damage and removal chemistries to ensure that each well is effectively cleaned. These tests can also be used to evaluate new fluid systems before taking them to the field. Our approach in this instance is to compare data from new fluids to results obtained with fluids with which we have a successful field track record. In this article we will review these procedures and the results with a variety of fluid systems. Introduction Shell completes horizontal wells in the unconsolidated sands of the Gulf of Mexico using openhole completions with sand control screens. In a limited number of cases, these wells have also been gravel packed. The sand control screens employed are generally pre-packed or contain sintered metal membranes with pore openings small enough to prevent formation sand production. For many Gulf of Mexico sands, this requires use of fine media (40/60 gravel equivalent or finer). Specialized fluids are used to drill the openhole section of the horizontal well. These fluids must perform the function of both drilling fluids (muds) and completion fluids. Drilling Issues. As drilling fluids they must provide well control (density), fluid loss control, lost circulation control [the equivalent circulating density (ECD) must be low enough to avoid fracturing the formation], adequate cuttings transport (rheology), hole stability (shale stabilization), rate of penetration, and ability to slide and steer (lubricity). The evaluation of fluids to meet these criteria is well established in the drilling mud industry1 and will not be discussed in detail in this article. Suffice it to say that drill-in fluids are tested using a standard suite of drilling mud evaluation procedures. In general, drill-in fluids do not perform as well as drilling muds in all areas [particularly rate of penetration (ROP) and lubricity], but the performance of a particular fluid must be adequate for each specific well. Completion Issues. As a completion fluid, the drill-in fluid (DIF) must provide sufficient density for well control and be nondamaging to the formation and completion. The fine-grained sandstones and small-pore-sized screen materials are both susceptible to plugging. Fluids must be designed and used in a manner that eliminates in-depth plugging of formation sands (formation damage) and plugging of the sand control screen. Industry data2,3 show that when the drill-in fluid is properly designed, formation invasion is limited to a few grain diameters into the formation face. One criterion for selecting a drill-in fluid is therefore that it forms a tight, competent external filtercake. As discussed further below and by others, these cakes are easily and completely removed from the formation face by flow4–6 and cleanup procedures.3,7-11 Completion procedures and fluids must also address concerns about plugging the sand control screen.12 In the first place, the drill-in fluid (solids laden) must be effectively displaced from the screen/openhole annulus before production is initiated. Special chemical sweeps and velocity requirements must be met to achieve this. Dissolution of the filtercake solids may also be required. In general, sand control screens are highly permeable compared to formation sands. Even when significantly plugged (e.g., 90%+), they add minimal drop in pressure to the overall well drawdown and may not contribute significantly to skin. Nonetheless, prior to development of the practices and procedures described here, we experienced high near-wellbore drawdowns on the order of 1,000 psi in several openhole horizontal wells. Since employing more systematic and aggressive cleanup procedures, we have not observed this phenomenon. Well Reliability Issues. A more serious concern with screen plugging is that it is not uniform and thus contributes to flow localization (hot spots) and erosional failure of sand control equipment. Given the high transmissibility of the screen/openhole annulus, a modest local decrease in screen permeability is enough to divert flow into unplugged portions of the screen. Since horizontal wells are generally highly productive, this flow localization can cause local velocities to exceed the critical velocities for erosion. This problem was implicated in the majority of sand control failures we experienced in horizontal wells. Since employing more systematic and aggressive cleanup procedures, we have reduced our sand control failure rate to an acceptable level. Fluid Selection Philosophy. Systematic cleanup of the openhole interval consists of selecting fluids that do not penetrate the formation and selecting cleanup systems that effectively remove the filtercake and fluid residues from the openhole and near wellbore. Comprehensive formation damage and cleanup evaluation is expensive and difficult to accomplish within the time constraints of a typical drilling program. We have thus focused on developing tests to assess the two main selection and design criteria: prevention of formation invasion and effective cake cleanup. The tests do not attempt to re-create downhole conditions but have been standardized on key conditions as will be discussed. Acceptance of a fluid is based on benchmarking against fluids and procedures that have been successful in the field. In this article we will discuss the two laboratory test procedures we use: return permeability and dynamic filtercake dissolution.
l%iipaporws wbdedfcw~ti by*n SPrOgram&mmitkw fdlMng review d infommfion cmtined in qn d%bacf subrntted by the author(s). Contenk d the paper, as m, ti m *~by the Society IX Petroleum Engineers and are subject to cotmdmn by the adtax(s). The material, as presented, does WI necesswify reflect any p.%ib!dfk Sod8tYd%bdMI Enginwm, its titers, or members Papa presented at SPE medngs qm sub+xi to pubkation review by Editorial Gmwndtees d tie Saiety of I%rdeum~neem. EbcbOnii mwod~, dsbibUtiOn, or stmge d any part d this paper for~W-~ti writfan consent d tie .Scciety of Petroleum Engineers is prcMi. Pennii to mpmdw in pint is mSrided to an abstract d nof mofe than XQ wrd$ Ilhstrati may nd be copied. The qbstract nwst contain conspicuous~d~m qnd byvhm the paper was presented Write Librarian, wE, P O -==% Riirdsom Tx 750&282e, USA., fax 01 -972-9S2.9425. AbstractSuccessfid completion of openhole horizontal wells requires removal of potential impairment from the near wellbore and formation fke. Impairment can reduce well productivity and lead to failure of sand control screens due to plugging and erosion. Shell has adopted the practice in the Gulf of Mexico of employing drill-in fluids with acid soluble bridging and weighting agents for all openhole drilling and extensive displacement and acid cleanup procedures during horizontal well completions. Drill-in fluid and filtercakes are removed with speciaIly formulated sweeps and by attainment of sufficient velocity to achieve effective displacement Filtercakes are removed with specially formulated washes applied using washcups or coil tubing to ensure that cleanup formulations contact filtercake materials. Drill-in fluids and clean-up systems must be specially formulated and evaluated for specific applications. We have adopted a suite of Iaboratoty screening tests to evaluate formation damage and removal chemistries to ensure that each well is effectively cleaned. These tests can also be used to evaluate new fluid systems before taking them to the field. Our approach in this instance is to compare data from new fluids to results obtained with fluids we have a successful field track record with. In this paper we will review these procedures and results with results with a variety of fluid systems.
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