ANNUAL TRANSACTIONS OF THE NORDIC RHEOLOGY SOCIETY, VOL. 12, 2004 On the Rheological Parameters Governing Oilwell Cement Slurry Stability Roni Gandelman, Cristiane Miranda, Kleber Teixeira, André L. Martins and Alex Waldmann PETROBRAS, Rio de Janeiro, Brazil. ABSTRACT positioning between the casing and the rock Cement slurry stability is a major formation. requirement for successful oilwell The main objective of the cementing job cementing, especially in high angle or is to assure zonal isolation avoiding flow of horizontal wells. The industry has provided fluids along the cement sheath. Others different solutions for the subject always objectives of the cementing job are to regarding the slurry response to support the casing and to protect it against standardized sedimentation and stability aggressive fluids existing in subsurface experiments. formations, to seal off abnormal pressure The purpose of the present work is to formations, to isolate incompetent analyze the rheological properties governing formations and to shut off lost circulation the sedimentation phenomenon. The zones. strategy adopted was to choose a few Cement slurries for oil wells are different slurry systems which behave composed by Portland Class G or H cement, differently regarding sedimentation and water and chemical additives, which may perform their complete rheological provide the required physical properties. characterization in shear flow. Rheology plays important roles on slurry Slurry compositions included design: conventional and high compacity systems. Rheological tests performed include low • In assuring that the cement slurry can be shear viscosity besides conventional oilfield mixed at the surface and can be pumped rheological analysis. The final purpose is to into the well with minimum pressure establish rheological design parameters for drop. different oilwell cementing operations. • In governing the flow regime for optimum cement slurry placement. INTRODUCTION • In maintaining the solid particles in The productivity of an oilwell is quite suspension during the fluid state of the affected by the cementing quality. The cement slurry. cementing job in an oilwell consists on pumping cement slurry into the well with The main goal of the present article is to proper physical and chemical properties establish the physical relations between which enables its displacement through a slurry rheological properties and its casing, previously run into the well, and its tendency to sediment. Slurry sedimentation free water channel will result in failures on is a major issue in highly deviated or zonal isolation, reducing dramatically the horizontal wells, where the formation of a productivity of the well (Fig. 1). Free wate r Rheology Cement After the sample preparation, the cement slurry is homogenized at a rotational speed of 150 rpm for 20 minutes in an atmospheric consistometer. The temperature is kept Formation constant at 80 °F. The apparatus used for the fluid test is a concentric cylinder device (Couette Flow) commonly used in the oilfield (Fann VG 35 A). After the homogenization, the Casing slurry is placed at the test vessel. The torque Figure 1. Free water channel response for each rotational speed provided by the equipment (300, 200, 100 and 6 rpm) A simplified test matrix is proposed (511, 340, 171 and 10 s-1, respectively) is including slurries of different chemical recorded. In fact the equipment also natures and compositions to be evaluated provides readings at 600 and 3 rpm, but they regarding sedimentation and rheology. A were not considered in this study for the brief analysis of the advantages and following reasons: there is controversy limitations of oilfield rheometer allows the concerning the guarantee of a laminar development of comprehensive guidelines rheometric flow at the higher speed for for slurry design. several slurries and there is frequently poor repeatability of readings at the lower speed. TEST PROCEDURES With the rotational speed adopted, shear Several experiments were run with the rates ranged from 10–511 s-1. After this, selected slurry systems aiming the initial and final gel strength measurements evaluation of their sedimentation and are performed, as follows: rheological behavior. Most experiments follow the American Petroleum Institute • The slurry is submitted to 600 rpm for 1 report1. Some additional rheological minute. experiments were run in order to establish • The rotor movement is ceased for 10 s. low shear rheology properties. A brief • The rotor is started at 3 rpm and the description follows. maximum torque value is recorded (initial gel). Preparation of Cement Slurry Samples • The rotor is ceased for 10 min. Dry materials are weighed and then • The rotor is started at 3 rpm and the uniformly blended before being added to the maximum torque value is recorded (final mixing fluids. The mixer motor is turned on gel). and maintained at 4000 ± 200 rpm. Water and fluid additives are then stirred at the Additional rheological experiments were above rotational speed to thoroughly performed: properties were obtained at the disperse them prior to cement addition. The rotational rheometer “Advanced Rheometric cement and solid additives blend should be Expansion System” (ARES, Rheometric added at an uniform rate, in not more then Scientific), with cone-and-plate geometry, 15 seconds. After the addition of all dry 50 mm diameter and a gap equal to 0.05 materials to the mix water, the mixing speed mm. is increased to 12000 ± 500 rpm for 35 In these experiments, it is possible to seconds. Dry materials (cement and solid evaluate low shear rate viscosity and additives) and water temperature should be viscoelastic behavior. For the present kept at 23.0 ± 1.1 °C prior to the mixing. article, only low shear viscosity results are standardized coil spring connecting the presented. Since time is an important stirring paddle and the stationary head. The variable on the slurry rheological properties cement slurry is kept in the container at this development, different tests were run with constant rotational speed and its consistency the same sample at different times and time dependency is recorded. The main mixing procedures. purpose of this test is to evaluate the evolution of apparent viscosity of the slurry Free Fluid with time. After the sample preparation, the cement slurry is homogenized with a rotational EXPERIMENTAL WORK speed of 150 rpm for 20 minutes in an Six different cement slurries were atmospheric consistometer. The temperature chosen to perform the analysis. The idea is kept constant at 80 °F. After that, the was to mix slurries with different slurry sample is poured into a clear formulations where different rheological graduated tube with a precision of 2 mL. and sedimentation properties were expected. The free fluid test slurry volume is 250 mL. A brief description of each one follows: The 2 hours test period is initiated when the conditioned slurry is poured into the • Slurry A – Contains a pozzolanic graduated tube at essentially vibration free material, dispersant, fluid loss additive conditions. The graduated cylinder is sealed and accelerator. Specific gravity = 1.67 with a plastic film wrap to prevent g/cm3. evaporation. Total water segregation is • Slurry B - Cement slurry containing recorded after 2 hours. microspheres, retarder and dispersant. Specific Gravity 1.62 g/cm3. 2 Sedimentation • Slurry C - Cement slurry containing After preparation, the cement slurry is microspheres, dispersant and retarder. poured into a sedimentation cylinder until Specific gravity = 1.38 g/cm3. the top. The internal diameter of the • Slurry D - Cement slurry with no sedimentation tube is 25 mm and its length additives, just water. Specific Gravity = is 200 mm. The slurry is cured for 24 hours. 1.87 g/cm3. After this time, the cement is removed from • Slurry E - Cement slurry containing the tube. The length of the set cement 3 retarder. Specific Gravity = 1.92 g/cm . specimen is measured. The sample is then • Slurry F - Cement slurry containing mechanically divided in four segments of 50 3 mm. The specific gravity of each segment is retarder. Specific Gravity = 1.89 g/cm measured and the difference between the top and the bottom segments is related to the These slurries were classified at an slurry sedimentation. increasing degree of free water or instability as detailed in Table 1. The time dependent behaviors of the Thickening time slurries were identified by two different A rotating cylindrical slurry container manners: equipped with a stationary paddle assembly essentially forms the equipment used in • Thickening time tests which, for the six these tests, named consistometer. The slurry slurries, showed no influence of time in container is rotated at a speed of 150 rpm the consistency index. This behavior is (2.5 rad/s) ± 15 rpm (0.25 rad/s). The explained by the addition of retarding consistency of the cement slurry is indicated additives in the slurry in order to prevent by the amount of deformation of a setting during testing time. Table 1. Sedimentation properties of each cement slurry. Free water Stability* Slurry (cm3) ∆ρ (g/cm³) A 0.0 0.0 B 0.5 0.036 C 1.6 0.012 D 2.0 0.024 E 4.0 0.048 F 8.0 0.060 *Stability: difference between the specific gravity of Figure 2. Viscosity response at different the top and bottom cement segment. times – Slurry A • Rheological tests at different times in order to capture hydration and sedimentation responses. All shear tests were run at an increasing and then decreasing ramp of shear rates. Such tests were run at the cone and plate geometry instrument according to the following schedule: o T1: test started immediately (1 min) after the end of slurry mixing. o T2: test started 50 min after the end of slurry mixing. No further homogenization was performed. o T3: test started 100 min after the Figure 3. Viscosity response at different end of slurry mixing. In this test, times – Slurry E. the slurry was re-stirred for 5 minutes immediately before the beginning of the test.