Mengjiao Yu

A study of wellbore stability in shales including poroelastic, chemical, and thermal effects

Abstract This paper presents the development of a model for determining wellbore stability for oil and gas drilling operations. The effects of mechanical forces and poroelasticity on shale behavior are included, as well as chemical and thermal effects. Chemical effects are caused by the imbalance between the water activity in the drilling mud and the shale water activity. The magnitude of this contribution depends on the effectiveness of the mud/shale system to perform as a semipermeable membrane. Experimental results show that osmotic pressures develop inside shales when they are exposed to different drilling fluids. This osmotic pressure is treated as an equivalent hydraulic potential, and is then added to the hydraulic wellbore and pore pressure as time progresses. Thermal diffusion inside the drilled formation induces additional pore pressure and rock stress changes and consequently affects shale stability. Thermal effects are important because thermal diffusion into shale formations occurs more quickly than hydraulic diffusion and thereby dominates pore pressure changes during early time. Rock temperature and pore pressure are coupled for most porous media studies; however, we have found that they can be partially decoupled for shale formations by assuming that convective heat transfer is negligible. The partially decoupled temperature and pore pressure effects can therefore be solved analytically under appropriate initial and boundary conditions. Experimental data for shale strength alteration, which occurs when shales are exposed to different fluids, are also included for the determination of cohesion strength decay. Pore pressure, collapse stress, and critical mud weights are variables investigated for determining poroelastic, chemical, and thermal effects on shale stability. The most important factors, which affect wellbore stability, are clearly identified.

Chemical and thermal effects on wellbore stability of shale formations

The model presented in Chapter 2 is compared with experimental data presented by Ewy and Stankovich [2000]. It is shown that the relative magnitude of the hydraulic conductivity of the shale (KI), the membrane efficiency of the shale (KII), and the effective diffusion coefficient of solute (Deff) all have an influence on the net pore pressure behavior of a shale exposed to a drilling mud. After the model has been calibrated with one set of experimental data, excellent predictions under other operating conditions can be made. Good agreement with experimental data is obtained for such predictions.

Chemical–mechanical wellbore instability model for shales: accounting for solute diffusion

Abstract A model that combines chemical effects with mechanical effects and provides a quantitative tool for evaluating wellbore stability is presented. In the past, wellbore stability models have introduced chemical effects by adding an osmotic potential modified by a membrane efficiency to the pressure acting at the wellbore wall [Fonseca, C.F., 2000. Chemical–mechanical modeling of wellbore instability in shales. Proceeding of ETCE 2000 and OMAE 2000 Joint Conference: Energy for the New Millenium, Feb. 14–17, 2000, New Orleans, LA.]. In this paper, an entirely different approach is adopted. The fluxes of water and ions into and out of the shale are accounted for. The pressure profiles obtained using our model differ significantly from the error function decline in pressure that is predicted by earlier models. As a consequence of this near wellbore pore pressure profile, wellbore failure can now occur inside the shale not just at the wellbore wall (as predicted by earlier models). The onset of instability now depends not only on the activity of the water but also on the properties of the solutes.

An Experimental Study of Hole Cleaning Under Simulated Downhole Conditions

With increasing measured depths and horizontal displacements in extended-reach, high-angle wells, hole cleaning remains one of the major factors affecting cost, time and quality of directional, horizontal, extended reach and multilateral oil/gas wells. This study involves experimental research and theoretical analysis to enhance cuttings transport capacity in oil and gas well drilling operations. The effects of drilling fluid rheology, mud density, temperature, borehole inclination, pipe rotation, eccentricity, rate of penetration (ROP) and flow rates were investigated experimentally. Volumetric cuttings concentration in the test section and frictional pressure losses were measured during the tests using two nuclear densitometers and a differential pressure transducer. A total of 116 experiments were conducted on a fullscale, Elevated-Pressure Elevated-Temperature Flow Loop (57.4 ft long, 5.76” x 3.5” annular section) at the University of Tulsa under controlled experimental conditions (up to 200 oF and 2,000 psi). Experimental results show that drill pipe rotation, temperature and rheological parameters of the drilling fluids have significant effects on cuttings transport efficiency. A dimensional analysis was conducted in this study to develop correlations that can be used for field applications. A user-friendly simulator was developed based on the results of the dimensional analysis and correlations. This simulator can be used by drilling engineers for design and sensitivity study. Results from this study can be used to determine critical conditions for efficient hole cleaning, as well as to optimize the mud program during the planning and operational phases of drilling.

Pressure Profile in Annulus: Solids Play a Significant Role

This paper looks into the effects of solids on the wellbore pressure profile under different conditions. An extensive number of experiments were conducted on a 90-ft-long, 4.5 in. 8 in. full-scale flow loop to simulate field conditions. The flow configurations are analyzed. A solid–liquid two-phase flow configuration map is proposed. Significant difference is found between the pressure profile with solids and without solids in the wellbore. The results of this study show how the pressure profile in the wellbore varies when solids present in the annulus, which may have important applications in drilling operations. [DOI: 10.1115/1.4030845]

Modeling Transient Circulating Mud Temperature in the Event of Lost Circulation and its Application in Locating Loss Zones

Lost circulation is one of the most persistent and costly drilling problems that drilling engineers have been struggling with for decades. The main reason why some of the remedial procedures are not working as planned is the lack of information, such as the location of the loss zone. The pinpointing of the zone of loss will allow the treatment to be applied directly to the point of loss rather than to the entire open hole.This paper presents an approach to predict the location of loss zone from the transient mud circulation temperature profile altered by the mud loss. A numerical model in estimating the transient mud circulating temperature profile during a lost circulation event is developed. The temperature profile in both the flow conduits (drillpipe and annulus) are modeled using mass and energy balance. The flow rate of drilling mud decreases in the annulus above the loss zone as part of the fluids lost into the fractures, which in turn alters the heat transfer between the drillpipe, annulus, and formation. The wellbore is divided into two multiple sections, which account for single multiple loss circulation zones. Rigorous heat transfer in the formation is included. Case studies are performed and numerical solution results are presented and analyzed. According to the results, temperature alterations induced by mud loss include: 1) Declines in both bottom-hole temperature (BHT) and mud return temperature over time, and 2) Discontinuity in the first order derivative of annulus temperature with respect to depth at the location of loss zone; meanwhile, the temperature alterations are mainly controlled by the mud loss rate and location of loss. By matching the simulated results with the distributed temperature measurements at different times, the depth of the loss zone can be identified. This piece of information is important for the spotting of LCM (lost circulation material) pills, the optimization of overbalance squeezing pressure, as well as the consideration of setting the cement plug or additional casing.Copyright

Determination of Cuttings Lag in Horizontal and Deviated Wells

A literature review, preliminary modeling, preliminary experimental work and recent development of this project are presented in this report. The literature review has been focused on cuttings transport in horizontal and slightly inclined well-bores, solidliquid flow patterns (two and three layer models), particle slip velocity and particle tracking. To model the phenomena, correlations developed by Larsen, Iyoho 12 and Chien were used to calculate the particle traveling velocity, and then it was used to associate the particles with their original location using other relationships. Preliminary experimental work has been completed using the Low Pressure Ambient Temperature Flow Loop (L.P.A.T.) and a high speed camera. Due to a recent development the current report includes a suggested modification of the flow loop in order to record the bed height in the inclined condition.

The Effects of Anisotropic Transport Coefficients on Pore Pressure in Shale Formations

This study investigates shale–fluid interactions through experimental approaches under simulated in situ conditions to determine the effects of bedding plane orientation on fluid flow through shale. Current wellbore stability models are developed based on isotropic conditions, where fluid transport coefficients are only considered in the radial direction. This paper also presents a novel mathematical method, which takes into account the three-dimensional coupled flow of water and solutes due to hydraulic, chemical, and electrical potential imposed by the drilling fluid and/or the shale formation. Numerical results indicate that the presence of microfissures can change the pore pressure distribution significantly around the wellbore and thus directly affect the mechanical strength of the shale.

Relation Between the Mechanical Specific Energy, Cuttings Morphology, and PDC Cutter Geometry

This paper discusses a series of experiments performed on Carthage Marble Limestone rock samples in a high pressure single PDC cutter testing facility. The tests were performed at 450 psi confining pressure conditions and four different cutters were used. Two different cutter diameter sizes of 13 mm and 16 mm, each with two different chamfer sizes of 0.010 inch and 0.016 inch were tested. Effect of the cutter geometry on the MSE of the cutting action and grain size distribution are discussed in this paper.The experimental results show that, in the tested range, the difference between the MSE when the two different cutter sizes are considered is insignificant. On the other hand, the results show that a change in the chamfer length from 0.010 inch to 0.016 inch can significantly increase the required MSE (as much as 20%). The cuttings produced at each test were gathered and tested in a particle size testing facility and the results were analyzed to determine which drilling parameters can be best correlated to the particle size distribution of the cuttings. The results show that the minimum particle size of the cuttings has a relatively strong dependency on the MSE of each test. The minimum particle size decreases as the MSE of the cutting increases and this is closely related to the extra energy required to regrind and crush the rock as a consequence of decreased drilling efficiency. In fact, the MSE can be estimated by the rock fracture surface energy and the grain size distribution.Copyright

Density and Drag Reduction With Hollow Glass Additives

This study concentrates on the use of materials known as hollow glass spheres, also known as glass bubbles, to reduce the drilling fluid density below the base fluid density without introducing a compressible phase to the wellbore. Four types of lightweight glass spheres with different physical properties were tested for their impact on rheological behavior, density reduction effect, survival ratio at elevated pressures and hydraulic drag reduction effect when mixed with water based fluids. A Fann75 HPHT viscometer and a flow loop were used for the experiments. Results show that glass spheres successfully reduce the density of the base drilling fluid while maintaining an average of 0.93 survival ratio, the rheological behavior of the tested fluids at elevated concentrations of glass bubbles is similar to the rheological behavior of conventional drilling fluids and hydraulic drag reduction is present up to certain concentrations. All results were integrated into hydraulics calculations for a wellbore scenario that accounts for the effect of temperature and pressure on rheological properties, as well as the effect of glass bubble concentration on mud temperature distribution along the wellbore. The effect of drag reduction was also considered in the calculations.Copyright