Slurry Column Research Articles

Cement Bonding Characteristics in Gas Wells

Summary A program was performed to study the effects of cyclic pressure/temperature fluctuations of gas storage wells on annular leakage. A review of pertinent literature was conducted, and operators of gas storage wells were surveyed to determine their experience in the area. Results indicated that for most of the gas storage wells currently in operation. Surface leakage is not a problem. Where leakage is reported, cyclic fluctuations do not appear to be a significant cause. In the majority of cases, leakage occurs within the first few cycles, indicating that it is caused more by static then by dynamic loads. The only variables found to correlate with leakage at a high level of significance were depth and bottomhole pressure (BHP). Wells that leaked tended to be deeper and had higher pressures than those that did not. The only way to stop leakage effectively at these higher pressures is the use of mechanical sealing mechanisms. Introduction and Background Gas intrusion into cemented wellbores and the resultant leakage to either the surface or porous formation below the wellhead have been persistent problems in the gas industry for many years. These problems have resulted in significant safety hazards and economic loss to operators. Through the efforts of a number of researchers in the past 2 decades, cementing techniques in general have been vastly improved and have helped reduce the magnitude of the problem. Little work has been done, however, to determine the effects of pressure/temperature cycling on the bonding characteristics of annular cement to casing. Data of this type are of particular importance to operators of gas storage wells because these wells operate on periodic injection/withdrawal cycles with associated pressure/temperature fluctuations. The current project was initiated in an effort to begin study of this problem. It was conducted in two phases. First, available literature was reviewed to identify previous work in cementing technology in general and in the area of annular leakage of oil and gas wells in particular. Then, operators of gas storage wells were surveyed to determine the magnitude of the annular leakage problem, to identify similarities or differences in wells with known annular leakage, and to establish typical environmental/operating parameters of gas storage wells. Literature Review A computerized search of the open literature was conducted by use of the COMPENDEX data base (Dialog Information Services Inc.). From this search, 24 references were selected for more detailed review. These works were thought to describe fairly well the general evolution of thought from the early 1960's until the present on the causes and means of prevention of gas leakage (also called gas migration and annular gas flow) in cemented wellbores.1–24 A more detailed review of the papers than given here can be found in Ref. 25. Past research efforts can be divided into two broad categories: methods and materials designed to minimize the leakage of formation pressures through the cement itself (i.e., the percolation of gas into the cement during setting and curing and the resultant creation of leakage paths) and methods designed to prevent gas migration at the cement/casing and cement/formation interfaces. Some researchers categorized these as two separate failure mechanisms, while others categorized them as separate cases of a single mechanism. In either case, it is evident that means taken to prevent the formation of leakage paths through the cement itself will also help prevent the formation of leakage paths at the interfaces. Additional preventive measures may be necessary, however, to minimize or to prevent leakage at the interfaces. Most research has concentrated on the prevention of leakage through the cement column itself, and one fairly well developed theory explains the need for prevention of these types of failures. This theory states that annular gas flow is caused by a hydrostatic pressure loss sometime between placement of the slurry in the wellbore and the development of sufficient static gel strength to resist the percolation of gas into it.24 Static gel strength is that internally developed rigidity in the slurry that resists a force placed on it. The gelling process will begin immediately after pumping has stopped and will continue until the cement develops a set. As gel strength increases, the cement column begins partially to support itself and its volume decreases owing to loss of filtrate to the formations and/or to hydration. Hydrostatic pressure caused by the slurry column lessens as the column begins increasingly to support itself and as volume changes occur. If volume changes are large enough and if the gel strength is not yet sufficient to resist gas intrusion, leakage to other producing zones or to the surface (directly or through a "leap-frog" approach from zone to zone) may occur.

Modified sedimentation‐dispersion model for solids in a three‐phase slurry column

AbstractSolids distribution data for a three‐phase, batch‐fluidized slurry bubble column (SBC) are presented, using air as the gas phase, pure liquids and solutions as the liquid phase, and glass beads and carborundum catalyst powder as the solid phase. Solids distribution data for the three‐phase SBC operated in a continuous mode of operation are also presented, using nitrogen as the gas phase, water as the liquid phase, and glass beads as the solid phase. A new model to provide a reasonable approach to predict solids concentration distributions for systems containing polydispersed solids is presented. The model is a modification of standard sedimentation‐dispersion model published earlier. Empirical correlations for prediction of hindered settling velocity and solids dispersion coefficient for systems containing polydispersed solids are presented.A new method of evaluating critical gas velocity (CGV) from concentrations of the sample withdrawn at the same port of the SBC is presented. Also presented is a new mapping for CGV which separates the two regimes in the SBC, namely, incomplete fluidization and complete fluidization.

Effect of low velocity liquid pulsations on some hydrodynamic characteristics of liquid and gas‐liquid fluidized beds

Externally applied vibrations or pulsations have been extensively studied as a means for improving transfer characteristics in two-phase systems (Baird, 1966; Baird and Garstang, 1972; Kim and Baird, 1976; Battacharya and Harrison, 1976). Similar studies for three-phase systems, e.g., three-phase fluidized beds, have been limited to examining the effect on solid-liquid mass transfer of bubble cycling caused by vertical oscillation of a slurry column containing entrained gas bubles (Lemcoff and Jameson, 1975). In three-phase fluidization, bubble coalescence predominates over bubble breakup for small-particle beds (e.g., D/sub RHO/ < 3 mm for sand fluidized by water and air), so that the desirably high particle surface areas and low fluidizing velocities associated with such beds are accompanied by lower gas holdups and hence inferior gas-liquid volumetric mass transfer coefficients, compared to those for large-particle beds (Epstein, 1981; Deckwer and Schumpe, 1983). It is therefore of some interest to determine whether the undesirable features of small-particle beds are somewhat mitigated by the imposition of deliberately imposed liquid pulsations. In the present preliminary study at low pulse velocities (<5.8 mm/s), measurements were made, with and without pulsing, of phase holdups for liquid-solid, gas-liquid, and gas-liquid-solid systems; of liquid phase axial dispersion for the gas-liquid-solidmore » systems; and of the hydrodynamic transition from a fixed to a fluidized bed for the liquid-solid system.« less

HYDRODYNAMIC CHARACTERISTICS AND GAS-LIQUID MASS TRANSFER IN A DRAFT TUBE SLURRY REACTOR

Fundamental characteristics of hydrodynamics and mass transfer have been measured in an air lift slurry reactor with a draft tube. The solid suspension capacity, i.e., the critical solid holdup, the gas holdup and the volumetric gas-liquid mass transfer coefficient were measured in the two draft tube columns of 0.1485 and 0.10 m in diameter. Four activated carbon beads ranging in size from 0.25 to 2.19 mm in average diameter were utilized as suspended solids in the experiments. The critical solid holdup in the draft tube slurry column is found to be much greater than that in the conventional bubble column. An empirical correlation is developed to account for the critical solid holdup behavior in the draft tube column. The gas holdup in the draft tube column agrees well with that in the bubble column. The overall gas-liquid mass transfer coefficient, k1awas measured by the oxygen probe method. The effect of solid holdup on k1a is found to be negligible in the present system. The empirical equation is devel...

Utilization of Fly Ash in Oil and Gas Well Cementing Applications

AbstractDuring cementing operations, the hydrostatic pressure of the cement slurry column is utilized to control the reservoir pressure. However, when the well extends through weak formations, it is often necessary to lighten the hydrostatic pressure of the slurry by lowering the density to avoid hydraulically fracturing the reservoir rock. Densities of cement slurries can be lowered by adding pozzolanic lightweight additives such as fly ash. Fly ashes have found utility in numerous oil and gas well cementing applications, namely: 1) fly ash can be dry-blended with regular Portland cements or hydrated lime for use in primary cementing; 2) lightweight cement compositions can be formulated by adding lime and finely ground quartz to fly ash/cement mixtures; and 3) an ultralight-weight pozzolanic microsphere cement, with densities as low as 0.96–1.64 g/cm3, can be formulated by blending Portland cements with hollow microspheres isolated from fly ash. This paper discusses the physical and chemical properties of each of these cement slurries, along with their relative advantages and disadvantages. Areas of future research are also discussed.