Dale E. Jamison

Geomechanics of Lost-Circulation Events and Wellbore-Strengthening Operations

Lost circulation, a major complication of drilling operations, is commonly treated by adding materials of various types, shapes, and particle-size distributions to the drilling mud. Generally known as wellbore strengthening, this technique often helps the operator to drill with higher mud gradients compared with that suggested by the conventional fracture-gradient or borehole-fracture-limit analysis. The underlying mechanisms through which a wellbore is strengthened, however, are not yet fully understood. This study explores these wellbore-strengthening mechanisms through an analytical solution to the related solid-mechanics model of the wellbore and its adjacent fractures. The provided solution is generic in that it takes into account the mechanical interaction of multiple fractures between one another and the wellbore under an arbitrary state of in-situ stress anisotropy. An additional generality in this solution arises from its unification and quantification of some solid-mechanics aspects of the previous hypotheses that have been published on the subject—i.e., stress cage, as well as the tip isolation and its effect on the fracture-propagation resistance. In relation to the stress-cage theory, the study investigates the wellbore-hoop-stress enhancement upon fracturing. The findings indicate that the induced hoop stress is significant at some regions near the wellbore, especially in the general vicinity of the fracture(s). However, given the strong dependency of wellbore stress on the mechanical and geometrical parameters of the problem, generalizing these results to the entire region around the wellbore may not always be trivial. The study also examines tip isolation, a common feature of fracture-closure and propagation-resistance hypotheses, through the analysis of partially reduced fracture pressures and a breakdown criterion, defined by the critical stressintensity factor of the formation rock.

Engineered LCM Design Yields Novel Activating Material for Potential Application in Severe Lost Circulation Scenarios

Particulate lost circulation materials (LCM) that work for severe-to-total losses are difficult, if not impossible, to find. Solutions that are effective for lower loss rates do not perform well at higher loss rates. Many LCM formulations have been used to treat severe losses, but their design and use has been more trial and error based mostly upon successful case histories. This paper describes the development of a combination of materials that is used in conjunction with other Engineered, Composite Solutions (ECS) to further enhance their performance. A novel combination of swelling materials, retarder, and fibers with a large aspect ratio is proposed as an activator that can be deployed with ECS typically available on the rig. The activator was designed and tested under conditions that qualitatively resemble severe lost circulation scenarios (large fractures). A shale swell meter was modified to qualitatively compare the swelling behaviour of different materials under different temperatures and retarder concentrations. A polyacrylamide-based swelling material was found to be sensitive to both temperature and retarder concentration. A newly sourced, potentially reservoir-friendly swelling material was found to be sensitive to temperature only. The activator-ECS combinations were tested for plugging capability with Permeability Plugging Apparatus (PPA) test equipment using different size tapered slots. Data from these modified PPA tests were used to determine the best combination of activator and ECS for plugging a particular-sized fracture simulated by the tapered slot. Field applications of systems that led to this proposed approach are discussed along with laboratory data comparing the swelling behaviour of different materials as related to mixing and pumping times. Introduction Lost circulation (Messenger 1981), the complete loss of drilling fluid in to the formation, is a perennial issue that translates to billions of dollars annually in NPT and cost of replacing the solutions. Solutions to arrest lost circulation, Lost Circulation Materials (LCM), are also well known and available. For many years, cost and local availability have been the primary driving forces behind the selection of LCMs. Solutions for drilling fluid losses sometimes also depend on formation type. In sandstone type formations, lost circulation through natural or artificial flow paths up to 2500 microns can be arrested with particulate LCMs. Numerous solution types and case histories can be easily found by a simple search on Onepetro. But dealing with lost circulation and providing solutions in formations with severe-total losses, requires planning, rigorous testing on existing solutions, and sometimes new solutions altogether. Due to various natural fractures or vugs, carbonate formations are another type which may require novel LCMs. Most of the time, particulate LCMs do not work in carbonate formations and some sort of chemical sealant solution is explored. On the other hand, increased demand for hydrocarbons has inspired the industry to explore hydrocarbon targets once considered unreachable. This has led to drilling deeper and in harsher environments. Lost circulation in such difficult environments just adds to the operator’s costs. In cases where both situations are present, like when drilling in highly fractured formations or in carbonate formations that are in deep, harsh environments, costs associated with lost circulation (NPT) increase dramatically. Hence it is prudent to invest time and resources in finding new LCMs, solutions, and LCM evaluation techniques that would help quantify LCM performance. Whitfill 2010, Kumar 2010, 2011, Savari 2011, 2012, and Kulkarni 2012, 2012 have demonstrated the advantages of looking into LCMs using new tools, new methods for testing, and new combinations of LCMs.

Modeling of Shale-erosion Behavior in Aqueous Drilling Fluids

Abstract Numerous shale-stability issues can occur while drilling with water-based muds (WBMs), including shale sloughing and cutting disintegration. These issues can be detrimental to the formation and pose difficulties with respect to rheology control, possibly reducing the rate of penetration (ROP). A “shale-erosion test” is a well-known laboratory test used to characterize the erosion of cuttings in WBMs. This paper documents a mathematical modeling tool known as an artificial neural network (ANN) used to model the erosion behavior of shale cutting in WBM. The ANN model establishes complex relationships between a set of inputs and an output based on computational modeling. For ANN modeling of shale-erosion behavior, the shale mineralogy and fluid composition constitute a set of inputs, while experimentally obtained “% erosion or % recovery” of the cuttings from the shale-erosion test represent the output. Experimental data for building the ANN model was obtained by performing approximately 150 standard shale-erosion tests using five different shales with varying mineralogy and WBMs with varying salt concentrations/types, shale stabilizers, and mud weights. For every test conducted, the input data (shale and fluid characteristics) and the output data (% recovery) was incorporated into the ANN model. The ANN model was then run to establish relationships between inputs and the output, which exhibited excellent correlation with R2 ≈ 0.85–0.90. The ANN model was successfully validated for an independent set of shale-fluid interactions. With the novel ANN model in place, erosion behavior of cuttings could be predicted in advance, thereby reducing the number of trials necessary in technical service labs. Mud engineers can use this model on a real-time basis as the shale chemistry varies with the depth of the formation drilling. The model could provide convenient measurement of fluid performance, enabling fluid optimization necessary to obtain desired shale behavior in advance, thereby minimizing drilling risks and costs associated with these oftentimes unpredictable shales.