Problems in Cementing Horizontal Wells

" Any casing eccentricity becomes critical In horizontal wells because ofIts effect on flow velocity distributions In the wellbore." Introduction Horizontal wells are completed by one of four methods:open hole,casing packers,slotted or perforated liner, orcemented casing or liner. Completion Methods 1 through3 generally are known as "drainhole" completions, Method 4, commonlycalled a "cased-hole" or "stimulated" completion, is the focusof this paper. Formations that ordinarily would require stimulation in a vertical wellshould be completed as a cased-hole, or stimulated, completion. Obtaining asuccessful cement job is essential in this type of completion. Parameters thatcould affect the overall cement Parameters that could affect the overall cementjob are similar to those for vertical completions, with an emphasis ondisplacement mechanics and cement slurry design. Fig. 1 represents a horizontal wellbore and the parameters that affect thecement job. A settled-solids channel is observed on the lower side of theannulus and a filter cake and gelled drilling fluid are found around thecircumference. Removal of solids, filter cake, and gelled drilling fluid fromthese two problem areas is critical to ensure proper problem areas is criticalto ensure proper cementing results. Decentralized casing compounds the problemof displacement because cement slurry and spacers tend to follow the path ofleast resistance and bypass the path of least resistance and bypass thenarrower side (bottom) of the annulus. Therefore, using the casing equipmentnecessary to provide maximum centralization is essential. Cement composition is also important to achieve a successful cement job. Thecement composition shown in Fig. 1 has permitted a top-side channel of freewater to form that could prevent the necessary confinement of a stimulationtreatment. Finally, to confirm the success of cementing or to determine requirementsfor remedial work if problems occur, the horizontal cement job must beevaluated with acoustic tools. Proper logging and log evaluation techniquesmust be considered. This paper addresses the following major areas of concern: hole-cleaningproblems and displacement mechanics, pipe centralization, cement slurry design, and evaluation of the cement job through acoustic tools. Hole-Cleaning Problems and Displacement Mechanics Displacement of drilling fluid from highangle and horizontal wellbores iscomplicated by additional factors present under these conditions. A low-side solids channel may form by deposition and settling of drilledsolids and drilling-fluid weighting material. A channel filled with solids willnot seal the annulus for the life of the well. Also, solids channels couldcause problems with communication and confinement of stimulation treatments. These channels should be prevented from forming if possible; however, onceformed, they can be removed. Thin flushes, maximum pump rates, hole-cleaningcasing attachments, and pipe movement can help remove solids. Drilling-fluidproperties must be controlled within specific properties must be controlledwithin specific ranges, particularly in highly deviated and horizontal wells. Besides the conventional parameters of yield point, plastic viscosity, parameters of yield point, plastic viscosity, fluid loss, and gel strength, thedynamic settling characteristics of the drilling fluid must also be considered. Several authors 1–3 indicate that drilling-fluid yield point is the criticalparameter that must be controlled to very specific ranges. One author suggeststhat the drilling fluid should have a yieldpoint/plastic-viscosity ratiogreater than one (with yield point in lbf/100 ft and plastic viscosity incentipoise). The drilling-fluid settling characteristic must be controlled bysome means to prevent solids settling and thus to allow an adequate flow pathfor placement of the cement slurry. placement of the cement slurry. Also, inhorizontal wells, an increased possibility of a narrow casing-to-wellborepossibility of a narrow casing-to-wellbore clearance on one side exists becauseof pipe eccentricity combined with gravitational forces. Inadequate clearanceon the narrow side of these wellbores can lead to uncemented portions of casingcircumference because of the excessive forces needed to move any material(solids or gelled drilling fluids) in this area. Any casing eccentricitybecomes critical in horizontal wells because of its effect on flow velocitydistributions in the wellbore. JPT P. 398