Previous work on annular gas flow 1~4 has shown that the behavior of cement between its fluid and set states is the controlling factor that may allow gas entry. This transition phase of cement previously has not been recognized in slurry design since its importance was not understood fully and since test procedures for its definition had not been presented or established.To predict the occurrence of annular gas flow and to design cement slurries capable of helping to prevent annular gas flow, it is necessary to define the slurry characteristics at the beginning and end of this transition period as well as the length of time of the transition state. Test techniques have been developed to study the start of this transition period. Additional tests have been conducted to define the condition of cement required to prevent gas entry. The test techniques developed to define this transition period of a cement slurry are described.Numerous job variables such as pumping time, placement time, slurry composition, and circulating temperature and pressure were investigated to evaluate their influence on transition time. A method for using transition time and static gel strength (SGS) development data to help predict annular gas flow and to evaluate annular gas flow control materials is discussed.
Summary A previous investigation of the effect of thickening times on early compressive-strength and gel-strength development has been expanded to include a wider range of well conditions, a greater variety of slurry types, and a more thorough evaluation of static gel strength. No positive relationship between thickening time and the onset or the rate of static-gel-strength development could be found. With few exceptions, all slurries tested developed values greater than 48 Pa [100 lbf/100 sq ft] gel strength in less than 20 minutes. Data presented show that times needed to develop a specific static gel strength are more closely related to the type of slurry than to thickening time. A calculation method is given for estimating shutdown factor from static-gel-strength data. New data confirm that 12- and 24-hour compressive strengths are not significantly reduced by reasonable increases in thickening time. The maximum single-stage cement interval without an unreasonable waiting-on-cement (WOC) time for specified compressive strength has been redefined in terms of interval temperature differences. Introduction Many aspects of oilwell cementing are important enough to warrant study. One area that needs attention is the procedures used in designing cement to be placed in the procedures used in designing cement to be placed in the well and the specific physical requirement of the cement for adequate downhole performance. The cement formulation is one of the few items that can be changed easily to suit existing well conditions, but only within limitations. Although the cement slurry is one of the most tested items pumped into a well, it is still one of the most variable. The basic design of a cement slurry starts with the determination of what general properties are needed for predicted well conditions. With the basic slurry formulation predicted well conditions. With the basic slurry formulation established, two physical properties need to be considered. The cement must remain fluid long enough to be pumped to its desired location downhole. Also, once the pumped to its desired location downhole. Also, once the cement is in place, it must set and develop an adequate compressive-strength value within a specific time period. Obviously, if a slurry cannot be pumped into place, period. Obviously, if a slurry cannot be pumped into place, its intended purpose would be, at most, academic. For this reason, the majority of laboratory time spent on testing oilwell cements is for testing thickening time to determine retarder concentration needed and to compare actual bulk-blend samples to laboratory blends. The thickening-time and compressive-strength tests account for most tests conducted on cement in field laboratories. Unfortunately, thickening-time and compressive-strength tests do not tell the whole story. The imperfect thickening-time test only simulates actual job conditions up to the predicted placement time. After test accuracy variation is allowed for, a thickening time longer than the placement time allows for some margin of safety, but only placement time allows for some margin of safety, but only for continuous pumping at a lower-than-predicted rate. Thickening-time safety factors do not relate directly to how long a slurry can remain static and still be moved after an inadvertent or intentional shutdown during placement. placement. Sabins et al. discussed the changes taking place during this time period. Tinsley et al. showed how these changes could contribute to annular gas flow. The physical property measured for this transition from the fluid physical property measured for this transition from the fluid to the solid phase was the static gel strength, and a static-gel-strength device for its measurement under high-temperature/high-pressure (HTHP) conditions was described. Rao et al. Showed the development of an ultrasonic test device that continuously measured compressive strength from initial set to any time of interest on a single sample maintained at HTHP conditions.
A model of a cemented annulus was used to evaluate the pressure transmitted and maintained by a static column of cement after placement and until set conditions were reached. Pressure is lost during cement hydration, resulting in annular gas flow. Laboratory and field tests were conducted on a new compressible cement capable of maintaining more stable annular pressure against the formation. Introduction Annular gas flow has been a problem in the industry for many years. One of the first recognized annular gas flow problems occurred during cementing of gas storage wells in the mid-1960's. The severity of this problem has varied from a nuisance factor to one that can be very significant. This paper reviews various types of annular gas flow in different areas of the country and considers the cost currently being spent in remedial cementing operations.Research on the problem of annular gas flow is not new. A, brief summary of previous research and field applications are included.Although much effort has been put forth on annular gas flow problems, until recently no new solutions with positive results have been found. One reason for this has been some lack of understanding of the phase behavior of cement slurries. A 4-year research program has led to the development of a new compressible cement system. Results of its application in several field areas are reviewed. Field History on Annular Gas Flow Various types of annular gas flow problems occur in many different producing areas. Following are a few of the problems encountered in different areas, which show its general nature. Houston Area - Offshore (High Island) Formations in the High Island area are Pleistocene sequences. In this area, annular gas now has been encountered on surface and conductor strings. The annular gas flow can result from shallow gas sands or sands that have been pressurized because of similar flow from higher-pressured zones at greater depths. Annular gas flow in some areas of this field are more pronounced on intermediate production strings and liners. The problem is not limited to deviated holes but, in some cases, may be aggravated by hole angle. In this area, it is common to have gas flow back to surface within 0.5 to 1.5 hours after the plug is bumped. This occurs on pipe strings that are cemented back to surface as well as those where cement is brought back only into the interannulus of the last cemented string.Uncontrolled gas flow from shallow zones or deeper high-pressure and high-deliverability zones outside the cemented surface pipe can cause gas blowout at some distance from the cemented annulus on the ocean floor. Often it has been necessary to bring primary cement back short of the surface and then conduct an annulus squeeze job on the cement top in an attempt to shut off gas flow to the floor of the platform. This, however, does not always prevent the loss of gas into shallow sands from the lower zones. A survey on remedial cementing operations in this area for the past several years indicates that cost has varied from $20,000 to $350,000/well. JPT P. 1427^
" 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
Summary This paper surveys cementing terminology for clarifying cement setting and its capability to control interzonal fluid flow. It also demonstrates the results of several test procedures designed to measure two or more cement properties simultaneously under conditions that simulate job applications. These procedures test the validity of theories and procedures for controlling annular gas flow, cement bonding, interzonal channeling, and intrazone isolation. From these results, guidelines may be developed for using laboratory data to determine times for temperature logs, perforating, and stimulation treatments. Existing laboratory equipment was modified and new equipment was designed to measure the interrelationships between static-gel-strength development, volume reductions, hydration temperatures, permeability changes, net plastic-state shrinkage, and compressive strength. Test results indicated that (1) hydration volume reduction (HVR) during the transition period is relatively small and shows a general correlation to unit volume cement content; (2) temperature increases from hydration show a rough correlation to static-gel-strength development; (3) permeability during static-gel-strength development decreases rapidly and shows a definite correlation to fluid-loss test values; (4) plastic-state shrinkage is only a very small part of the total HVR; (5) a general correlation exists between compressive strength, HVR, and heat of hydration; (6) the HVR for an expansive cement was greater than that of a nonexpansive cement, but its plasticstate shrinkage was less. plasticstate shrinkage was less. Introduction Only a cursory review of current gas-migration and cement-bonding theories is needed to discover a serious lack of standardization of terms and definitions used for cement properties. This is especially true for changes occurring during the fluid to solid transition period and early strength development periods. period and early strength development periods. Even when some consensus on definitions exists, there is often no clear explanation of how one specific property relates to another. Many gas-migration and interzonal-communications control methods and materials are based on the assumption that changes in certain physical properties are somehow directly responsible for any job-quality improvements. With little consideration for other properties and events concurrent with the measured properties, properties and events concurrent with the measured properties, theories have been expanded that give major credit to minor itemse.g., gas permeability of the cement matrix, mixing-water density, inflow restrictions from filter cake, and, to some extent, gasdispersion (foaming) properties of the cement. Cement-bond improvement theories suffer a similar malady. Theories that concentrate on one property while neglecting changes in other properties often do more harm than good. For example, cement expansion is often overemphasized, while the influence of HVR, plastic-state shrinkage, fluid loss, and fluid inflow are neglected. Most of the common terms have been used for years and need only to be clarified to establish their definitions and to discourage conveniently changing their meaning to explain a new theory. Such terms include plastic-state shrinkage, initial set, final set, bulk volume, total solids volume, HVR, porosity, permeability, heat of hydration, and expansion. Definitions for these terms are mostly borrowed from concrete technology; to apply them to deep-well cementing, specifications regarding application temperature and pressure must be included. pressure must be included. The terms used to describe the chronology of deep-well cementing are more esoteric: mixing, fluid, placement, dormant transition, and hardening periods. Common to the transition period are such terms as static time, zero gel time, thixotropic gel period, nonominal transition period, and hydration gelation period. The hardening period (as applied to deep-well cementing) is from the first solid properties to initial set, final set, and ultimate strength. This paper clarifies the more accepted definitions for general cementing terms. The definitions of the chronological terms are based on typical usage in the literature and internal company reports and bulletins. The "specialized" terms include those originating in published works but are based largely on our experience with published works but are based largely on our experience with scalemodel cement tests and the development of laboratory testing equipment.
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