This article focuses on interpretation of well test data from wells completed in naturally fractured reservoirs. Because of the presence of two distinct types of porous media, the assumption of homogeneous behavior is no longer valid in naturally fractured reservoirs. This article discusses two naturally fractured reservoir models, the physics governing fluid flow in these reservoirs and semilog and type curve analysis techniques for well tests in these reservoirs. Naturally fractured reservoirs are characterized by the presence of two distinct types of porous media: matrix and fracture. Because of the different fluid storage and conductivity characteristics of the matrix and fractures, these reservoirs often are called dual-porosity reservoirs.

ABSTRACT A ship moving in water experiences resistances, the major component of which is frictional resistance arising from water viscosity. Therefore, drag-reduction has been an active research field in marine transportation over the last decades. A variety of innovative drag reduction techniques have been proposed, among which are riblets, microgrooves, compliant coatings, addition of polymers, air injection method, to name a few. Among them, air-injection drag reduction technique has achieved attraction increasingly in recent years owing to its overwhelming advantages, such as considerable potential drag reduction, easy operations, environmental friendliness and low costs. There are basically two types of air-injection drag reductions: Microbubble Drag Reduction (MBDR) and Air-Layer Drag Reduction (ALDR). It is interesting that they can be modeled by Eulerian-Eulerian two-fluid model and VOF (Volume of Fluid), respectively in OpenFOAM. In this paper, we show how to numerically model the two types of air-injection techniques. After that, a comparison with a recently experimental data is made for an axisymmetric body. It is surprising that such comparison reveals that MBDR and ALDR are all present in the experimental test. MBDR is dominant for low flux rate while ALDR is dominant for high flux rate. Their transition is restricted to a very narrow region, similar to a phase transition characterized by a sharp jump from one mechanism to another. Based on such observations we propose a combined numerical model based on the data-coupling method to handle the case that both MBDR and ALDR are present with a sharp transition. The combined model is a linear one in the framework of interpolation. It works well with satisfactory results obtained. Numerical results show that in the MBDR-dominant region the air void fraction around the body is the determinant factor influencing the drag reduction. In the ALDR-dominant region the formation area of the air layer around the body surface determines the amount of the drag reduction. The above results are useful for designing an effective air-injection drag reduction method.

Abstract As advanced drilling technology gains more popularity in the development of unconventional reservoirs and deepwater fields, the demand for an improved drilling hydraulics modeling escalates today. The situation is also true for foam applications that have long been used as a useful means of downhole cleaning and underbalanced drilling methods. This study investigates how a new foam model, recently developed by Wang et al. (2017), can be applied to a wide range of hole cleaning and drilling scenarios. The model combines rheological properties of wet foams and dry foams by using 9 model parameters (3 to define the range of gas and liquid flowrates and corresponding frictional pressure losses of interest, 4 to fit the power-law rheology of wet and dry foams, and last 2 to capture the sensitivity of foam rheology to gas and liquid rates). These scenarios consider foam circulation into 10,000 ft long wells at different inclination angles with a long vertical, inclined, or horizontal trajectory. The results from this new method are compared with two existing foam modeling techniques, so-called Chen et al.'s model (based on the correlations for wet foams only) and Edrisi and Kam's model (based on wet- and dry-foam rheological properties with 5 model parameters). The results show that, with or without formation fluid influx, the new foam model demonstrates the robustness of the new modeling technique in all scenarios capturing foam flow characteristics better, whenever the situation forms stable fine-textured foams or unstable coarse-textured foams.

Abstract Class I and Class II waste re-injection are the most important methods for disposing of fluid in North Dakota: in 2007, more than 96% of produced water were disposed of using underground injection, and by 2012 all produced water was being managed by underground injection. While Class II injection covers waste produced from most Exploration & Production (E&P) activities, Class I injection wells are used for disposing of a special class of industrial wastes, including waste generated by petroleum refining, metal production, chemical production, pharmaceutical production, commercial disposal, and food production. Non-hazardous industrial waste and Naturally Occurring Radioactive Materials (NORM) not associated with E&P can also be injected using Class I wells. In all cases, the primary concern for permitting and safe operations is to (1) predict the movement of the injected waste to ensure that it stays within pre-defined formations, and (2) ensure that pore-pressure increases caused by injection do not impact neighboring offset wells. Results from a geochemical study of the feasibility of disposal into the Dakota Sands (Inyan Kara formation) in North Dakota will be presented. Analyses were made using a compositional reservoir simulation (REVEAL) to predict the pore-pressure distribution, direction and movement of the injected fluid, as well as chemical reactions between formation brine/waste/formation rocks and the effect of these chemical reactions on formation injectivity and cap rock integrity. Forecasts indicate that over 50 years of injection the injected wastes will be completely trapped within the Dakota Sands (no fluid flow is expected to penetrate through the cap rock) and injection pressures are expected to remain well below the estimated fracture pressure. While the Inyan Kara formation is therefore a reasonable storage trap for industrial wastes, carbonate and sulfate scales may cause near wellbore formation damage and rising wellhead pressures which operators will need to address.

Abstract An experimental study followed by comprehensive flow modeling is presented. The experiments were conducted on a horizontal well setup with drillstring under compression, considering the influence of rotation on frictional pressure losses of Yield Power Law (YPL) fluids. Flow through various buckling configurations with and without drillstring rotation was investigated. A new correlation is presented for the transition from laminar to turbulent regions in concentric and eccentric annuli. A broad model of flow of YPL fluids is proposed for concentric, eccentric and buckled configurations. The model includes the effects of rotation in laminar, transitional and turbulent flow. A 91 ft. inner pipe was rotated while applying axial compression during flow. At the no-compression case, eccentricity of the inner pipe is varied as the drillstring rotated. The aim for such a design was to simulate actual drilling operations. The test matrix involves flow through sinusoidal, transitional and helically-buckled drillstring. The effect of pitch length is investigated. Helical modes with two different pitch lengths were tested. Eight distinct YPL fluids were used to examine the dependence of pressure losses on fluid parameters. In the theoretical part, a stability criterion is modified to determine the onset of transitional flow of YPL fluids and a correlation is proposed for practical purposes. In addition, pressure loss prediction models are presented for the flow of YPL fluids through concentric, eccentric, free and buckled configurations of the drillstring, with and without rotation. The proposed models are compared with data from the literature and the experiments. It has been observed that increasing eccentricity and rotation causes an earlier transition from laminar to turbulent flow. Increasing eccentricity decreases pressure losses. In addition, the buckled configurations show a further decrease in frictional pressure losses as the compression increases. In the helical mode, decreasing the pitch length results in a decrease in pressure losses. Rotation tests with free drillstring show an increase in pressure losses as the rotary speed of the drillstring increases. Also, rotating the drillstring while it is compressed suggests an elevated increase in pressure losses due to amplified vigorous motion of the drillstring. Distinct differences in the effects of buckling and rotation are observed in laminar, transitional and turbulent flow. The greatest differences are found in the transition region. Flow of YPL fluids is one of the greatest challenges in the modern drilling industry. Studies that correspond to actual drilling conditions are substantially important in reducing these challenges. The information obtained from this study can be used to improve the control of bottomhole pressure during extended reach (ERD), horizontal, managed pressure (MPD), offshore and slim hole drilling applications. Consequently, this theoretical and experimental research has the potential to lead to safer, deeper and more precisely controlled oil/gas well drilling operations.

Abstract Buildup data from multiply-fractured horizontal wells (MFHW) in tight (micro to nano-Darcy permeability) unconventional reservoirs often display a derivative signature which appears like radial flow and is frequently misinterpreted as such without due consideration of the expected flow regimes from these systems. Studies based on both numerical and analytical modeling suggest that radial flow regimes will not be observed, due to the large number of hydraulically induced fractures, and the geometry of a typical MFHW completion. Analysis of production data continues to show no evidence of radial flow, but instead, reveals that linear flow regimes dominate for the majority of a well's lifetime. This is more consistent with the expected flow behavior, and leads to confusion on how to interpret buildup data that often appear like radial flow. This paper provides explanations for the different pressure behavior exhibited during a buildup. In doing so, the value of conducting a buildup test to assess the reservoir characteristics and completion effectiveness, is demonstrated. Two examples of actual buildup data obtained from MFHWs completed in unconventional reservoirs are presented to support the findings of this paper.

Abstract A simplified dynamic model based on ordinary differential equations that predicts the surge and swab pressures in tripping operations is developed. It is fast and robust, and designed for application in real-time operations where the pressure surges must be predicted and controlled. The drilling mud is described as a Herschel-Bulkley non-Newtonian drilling fluid. And simplified continuous flow equations are used for fast computation of frictional pressure gradients. The model can readily be coupled with parameter estimation techniques, to automatically adapt uncertain parameters to measured data, or it can be calibrated manually as we show in this paper. This can be done during field operations if a minimum amount of measuring data are available.