Subhash N. Shah

Analytical modeling of gas recovery from in situ hydrates dissociation

Exploration activities around the world have confirmed that in situ hydrates are widely available in permafrost and oceanic sediments. These hydrocarbon deposits have stimulated worldwide efforts to understand gas production from hydrate dissociation in a porous media. This work is a contribution towards these efforts. The present study uses depressurization-driven in situ hydrate dissociation, incorporates decomposition kinetics into the radial diffusivity equation, and develops a model to predict the performance of naturally occurring hydrates. The proposed model is simple yet useful and it does not require any empirical correlation. The model-predicted performance compares well with the published experimental studies on the hydrate dissociation in porous media.

Correlating viscoelastic measurements of fracturing fluid to particles suspension and solids transport

An experimental study was performed to understand the relationship between fluid characteristics and its solids transport and sand suspension capabilities. In this study, borate crosslinked guar gels were prepared at three different pHs. The linear guar was crosslinked with borate crosslinker at concentrations from zero to the values where the gel exhibited phase separation at each pH. These gels were then characterized for viscoelastic properties and suspension settling velocities. At each pH, a crosslinker concentration was selected to evaluate the gels for their solids transport behavior in two large size slot models. The gels that satisfactorily transported solids through the slots had similar elastic moduli. The results show that the fluid elasticity, not viscosity, correlates with the solids transport capability of the crosslinked guar gels. Furthermore, the drag coefficients for the suspensions settling in the crosslinked gels prepared at three pHs were dissimilar, even at similar Weissenberg numbers. The drag on the suspensions settling in the viscoelastic guar gels was observed to be both more and less than the drag on the suspensions settling in the Newtonian fluids. Further work is needed to better understand these drag behaviors on the suspensions settling in viscoelastic guar gels.

A Three-Layer Modeling for Cuttings Transport with Coiled Tubing Horizontal Drilling

A new approach of three-layer flow of two-phase (solid-liquid) fluid in annulus, i.e. a stationary bed of drilled cuttings at bottom; a moving bed layer above it: and a heterogeneous suspension layer at the top, is proposed to predict and interpret the cuttings transport mechanism. This new mathematical model is presented to overcome the limitation in the existing two-layer models. This enables efficient prediction of cuttings transport during horizontal drilling using coiled tubing. In particular, the model described in this paper formulates transport process and includes the relevant parameters such as rheological characteristics of the drilling fluid, cuttings size/sphericity/ concentration, wellbore geometry, eccentricity of the coiled tubing, and pumping rate of the drilling fluid. This paper presents the model development, solution. and simulation of the cutting transport process to illustrate the steps involved in determining the effect of each parameter during the drilling operation. Graphical charts and developed correlations between annular flow rate, rheology, wellbore geometry, eccentricity, cuttings size and concentration in each layer, and the carrying capacity are also presented. The simulation results show how to obtain a reasonable pumping velocity of drilling fluid with the possible lowest pressure gradient that might serve as an operational guideline during drilling. Furthermore, the effects of parameters affecting the efficiency of cutting transport are discussed. These results are compared with published experimental data. The observed agreement and discrepancies are discussed, and further improvements on the current model are proposed.

Proppant Settling Correlations for Non-Newtonian Fluids Under Static and Dynamic Conditions

This study presents a new approach for analysis of proppant settling data in non-Newtonian pseudoplastic fracturing fluids and develops drag coefficient correlations as a function of fluid model parameter n/prime/. Results of experiments with these fluids under static as well as dynamic conditions are discussed. A wide range of n/prime/ and particle Reynolds number is investigated. It is shown that at low-particle Reynolds numbers the fluid model parameter n/prime/ has a significant effect on proppant settling velocity. This effect diminishes at higher particle Reynolds numbers. The dynamic settling velocity data agree reasonably well with the correlations developed from static velocity data. 18 refs.

Experimental Investigation on the Rheology of Foams

In this study, the rheology of foams was investigated using aqueous foams and gelled foams and employing a pipe type viscometer. Water was used as the liquid phase for the aqueous foams while guar was used as the gelling agent for the gelled foams. Surfactant at 0.5% (vol ) concentration was used as the foaming agent. The flow data showed that foams behave like Herschel-Bulkley model fluids New empirical correlations have been developed from the experimental data to predict foam fluid apparent viscosity. These correlations are fimction of liquid phase properties and foam quality and are applicable to the foam systems tested in the wide range of shear rates and temperatures investigated. The use of these new correlations will provide more accurate estimation of the foam fluid rheological properties.

Rheological Characterization of Hydraulic Fracturing Slurries

Few studies have dealt with the flow behavior of concentrated suspensions or slurries prepared with non-Newtonian carrier fluids. Therefore, the purpose of this investigation is to present experimental results obtained by pumping various hydraulic fracturing slurries into a fracture model and gathering data on differential pressure vs. flow rate. Several concentrations of hydroxypropyl guard (HPG), a wide range of proppant concentrations, and three test temperatures were studies. The effects of such variables as polymer gelling-agent concentration, proppant concentration, test temperature, and fracture-flow shear rate on the rheological properties of slurries were investigated. The correlations for predicting the relative slurry viscosity for these HPG fluids are presented. Substantial increases in viscosity of fracturing gels were observed, and results are discussed in light of several affecting variables. Results also are compared with those available for Newtonian and non-Newtonian concentrated suspensions. Applications of these correlations to estimate the hindered particle-settling velocity in the fracture caused by the presence of surrounding particles also is discussed. The correlations presented can easily be included in any currently available 2D or 3D fracture-design simulators; thus, the information can be applied directly to predict fracture geometry and extension.

New friction correlation for cements from pipe and rotational-viscometer data

This paper presents the results of 24 discrete slurries tested on a pipe flow loop while data were taken simultaneously with a flow-through rotational viscometer. Each slurry was pumped through four different pipe sizes at rates from low laminar to high turbulent flow regimes. Test slurries varied from very thixotropic to highly dispersed and from lightweight to dense. Analysis of pipe-flow data showed a correlation best described by a Bingham-plastic model. Standard flow properties calculated from the rotational-viscometer data were correlated to flow properties derived from the pipe-flow data. These results could be used to calculate pipe pressure losses with improved accuracy. Correlations for estimating friction losses of cement slurries in laminar and turbulent flow regimes and for predicting laminar/turbulent transition were developed. Results of these correlations are compared with the findings of previous investigators. An example illustrates the use of these correlations.

Breakdown Pressure Determination - A Fracture Mechanics Approach

Accurate determination of breakdown pressure in the presence of pre-existing fracture (e.g., natural fractures or perforations) can assist engineers better manage expected fracture gradients. The classical models by Hubbert and Willis, and Haimson and Fairhurst did not account for the existence of pre-existing fractures in predicting breakdown pressure. In addition, the available fracture models for the calculation of breakdown pressure do not consider nonlinear internal pressure distribution in the pre-existing fracture. Finally, some of them either ignored near wellbore stress concentrations, or they are limited to specific fracture dimensions. To overcome the limitations of current methodologies, a weight function method is applied to predict breakdown pressure of two general symmetrical radial fractures emanating from a borehole. A weight function parameter table and three weight function parameter correlations are provided for continuous dimensionless crack lengths from 0.001 to 100. For uniform pressure distribution in the pre-existing fracture, the weight function based breakdown pressure is compared against the PSA method [Paris and Sih, 1965; Abou-Sayed et al., 1978], the results show a good agreement. Weight functions are applied to predict breakdown pressure for uniform and nonuniform pressure distribution in a pre-existing fracture, and the results prove that the pressure distribution affects the breakdown pressure. Sensitivity studies are conducted to investigate the influence of pre-existing crack length, orientation, in-situ stress contrast, and fracture toughness on breakdown pressure. It indicates that breakdown pressure (1) does not always increase with increasing dimensionless crack length at different stress contrast, and (2) increase with increasing absolute value of deviation angle and fracture toughness. The weight function based breakdown pressure is further verified against measured breakdown pressure from laboratory hydraulic fracturing experiments by finetuning fracture toughness. The results are also in good agreement for selected successful fracturing experiments.

Investigation of flow behavior of slickwater in large straight and coiled tubing

This paper (SPE 118949) was accepted for presentation at the SPE Hydraulic Fracturing Technology Conference, The Woodlands, Texas, USA, 19–21 January 2009, and revised for publication. Original manuscript received for review 13 September 2008. Revised manuscript received for review 16 March 2009. Paper peer approved 31 March 2009. Summary Low-damage fracturing fluids are normally used for better fracture-dimension confinement and lower residue. This leads not only to longer fracture lengths, but also to higher fracture conductivity. Slickwater fracturing technology, developed in the 1980s, is less expensive than gel treatments. Fluid and proppant volumes can be reduced, and treatment flow rates can be increased significantly. When compared to conventional gel treatments, slickwater fracturing can generate similar or better production responses. In frac treatments, slickwater is pumped through straight tubing (ST) and coiled tubing (CT). As a result of secondary flow, frictional pressure losses in CT are higher than in ST. Determining these losses is of the utmost importance for successful treatments. Customarily, laboratory-generated flow data are used to develop correlations to predict frictional pressure loss of fluids flowing in ST and CT. These correlations are then, without verification, applied to field applications, which employ much larger conduit sizes. The present study is aimed to experimentally investigate the hydraulic properties of a commonly used drag reducer—Nalco ASP-820—in larger tubing sizes. This is a modified partially hydrolyzed polyacrylamide (PHPA). Previously, we reported flow data gathered employing 1⁄2-in. ST and CT (varying curvature ratio) and the correlations developed to predict frictional pressure losses of fluids in ST and CT. In this investigation, the large-scale flow data acquired from 200-ft ST and 1,000-, 2,000-, and 3,000-ft, 11⁄2-in. CT, and 1,000-ft 2⁄8and 2⁄8-in. CT, are compared with the predictions from the correlations developed from 1⁄2-in.-pipe data. The effects of shear intensity, shear duration, and pipe roughness on flow properties of the ASP-820 fluid are also investigated. Results show significantly lower drag reduction in CT than in ST because of secondary flow caused by the CT curvature. Polymer degradation because of shear (shear intensity and duration) and/or tubing roughness can reduce drag reduction significantly. The results are discussed in light of a recently developed “dragreduction envelope” to evaluate the drag reduction characteristics of slickwater fluid in ST and CT.

Rheological Properties and Frictional Pressure Loss of Drilling, Completion, and Stimulation Fluids in Coiled Tubing

The rheological properties and friction pressure losses of several common well-drilling, completion, and stimulation fluids have been investigated experimentally. These fluids include polymeric fluids-Xanthan gum, partially hydrolyzed polyacrylamide (PHPA), guar gum, and hydroxyethyl cellulose (HEC), bentonite drilling mud, oil-based drilling mud, and guar-based fracturing slurries. Rheological measurements using a Bohlin CS 50 rheometer and a model 35 Fann viscometer showed that these fluids exhibit shear thinning and thermal thinning behavior except the bentonite drilling mud whose viscosity increased as the temperature was raised. Flow experiments using a full-scale coiled tubing test facility showed that the friction pressure loss in coiled tubing is significantly higher than in straight tubing. Since the polymeric fluids displayed drag reducing property, their drag reduction behavior in straight and coiled tubings was analyzed and compared. Plots of drag reduction vs. generalized Reynolds number indicate that the drag reduction in coiled tubing was not affected by polymer concentration as much as in straight tubing. The onsets of turbulence and drag reduction in coiled tubing were significantly delayed as compared with straight tubing. The effect of solids content on the friction pressure losses in coiled tubing is also briefly discussed.