Robello Samuel

Realtime Rate of Penetration Optimization Using the Shuffled Frog Leaping Algorithm

The increasing complexities of wellbore geometry imply an increasing well cost. It has become more important than ever to achieve an increased rate of penetration (ROP) and, thus, reduced cost per foot. To achieve maximum ROP, an optimization of drilling parameters is required as the well is drilled. While there are different optimization techniques, there is no acceptable universal mathematical model that achieves maximum ROP accurately. Usually, conventional mathematical optimization techniques fail to accurately predict optimal parameters owing to the complex nature of downhole conditions. To account for these uncertainties, evolutionary-based algorithms can be used instead of mathematical optimizations. To arrive at the optimum drilling parameters efficiently and quickly, the metaheuristic evolutionary algorithm, called the “shuffled frog leaping algorithm,” (SFLA) is used in this paper. It is a type of rising swarm-intelligence optimizer that can optimize additional objectives, such as minimizing hydromechanical specific energy. In this paper, realtime gamma ray data are used to compute values of rock strength and bit–tooth wear. Variables used are weight on bit (WOB), bit rotation (N), and flow rate (Q). Each variable represents a frog. The value of each frog is derived based on the ROP models used individually or simultaneously through iteration. This optimizer lets each frog (WOB, N, and Q) jump to the best value (ROP) automatically, thus arriving at the near optimal solution. The method is also efficient in computing optimum drilling parameters for different formations in real time. The paper presents field examples to predict and estimate the parameters and compares them to the actual realtime data.

Modeling and Analysis of Drillstring Vibration in Riserless Environment

Riserless drilling poses numerous operational challenges that adversely affect the efficiency of the drilling process. These challenges include increased torque and drag, buckling, increased vibration, poor hole cleaning, tubular failures, poor cement jobs, and associated problems during tripping operations. These challenges are closely associated with complex bottomhole assemblies (BHAs) and the vibration of the drillstring when the topholes are drilled directionally. Current methods lack proper modeling to predict drillstring vibration. This paper presents and validates a modified model to predict severe damaging vibrations, analysis techniques, and guidelines to avoid the vibration damage to BHAs and their associated downhole tools in the riserless highly deviated wells. The dynamic analysis model is based on forced frequency response (FFR) to solve for resonant frequencies. In addition, a mathematical formulation includes viscous, axial, torsional, and structural damping mechanisms. With careful consideration of input parameters and judicious analysis of the results, the author demonstrates that drillstring vibration can be avoided by determining the 3D vibrational response at selected excitations that are likely to cause them. The analysis also provides an estimate of relative bending stresses, shear forces, and lateral displacements for the assembly used. Based on the study, severe vibrations causing potentially damaging operating conditions were avoided, which posed a major problem in the nearby wells. The study indicates that the results are influenced by various parameters, including depth of the mud line, offset of the wellhead from the rig center, wellbore inclination, curvature, wellbore torsion, and angle of entry into the wellhead. This study compares simulated predictions with actual well data and describes the applicability of the model. Simple guidelines are provided to estimate the operating range of the drilling parameter to mitigate and avoid downhole tool failures.

3D Geomechanical Modeling of Salt-Creep Behavior on Wellbore Casing for Presalt Reservoirs

Exploration drilling is venturing out into deeper regions of water. While exploring these deeper water depths, large hydrocarbon deposits have been found below salt formations. These reservoirs are located in formations called “pre-salts,” which are located below the salt formations. Pre-salt reservoirs have been found in offshore Brazil, the Gulf of Mexico, West Africa, and the North Sea. Completions in salt formations can be difficult owing to the creep behavior that the salt formations exhibit. Creep behavior results from the instability of the salt formation, which causes a slow flow and permanent deformations. Creep deformation occurs over time and is initiated once the salt formation has been penetrated. Completion of the wellbore does not stop formation creep. The constant creep of the salt formation causes excess stress on the wellbore casing, which may eventually cause the casing to collapse. In this study, a 3D geomechanical model is developed, using data such as wellbore pressure and temperature, formation stress and temperature, rock, cement, and casing properties, to predict the effects of salt creep behavior on stress loading in the wellbore casing, which helps to assess the life expectancy of wells in pre-salt reservoirs. The simulation results of this model can provide quantitative results of casing stress and deformation as a function of time under various temperature, in-situ stress and operation conditions, that can be used as useful information for subsequent wellbore casing design and wellbore integrity analysis. In addition, possible methods that can mitigate the severity of salt mobility and reduce the risks of casing collapse are discussed.

Analytical Model to Estimate the Downhole Casing Wear Using the Total Wellbore Energy

The increasing complexities of wellbore geometry imply an increasing potential of damage resulting from the casing-wear downhole. Much work has been done to quantify and estimate wear in casing; however, the results of such predictions have been mixed. While the locations of critical-wear areas along the casing string have been predicted fairly accurately, quantifying the actual amount of casing wear has been a magnitude off. A mathematical model that describes this casing wear in terms of the total wellbore energy has been developed and used to estimate the depth of the wear groove and the wear volume downhole. The wellbore energy provides a mathematical criterion to quantify the borehole quality and incorporates the parameters, borehole curvature, and the wellbore torsion. The casing wear observed downhole is also an integral function of these two parameters. Hence, a combined “wear-energy” model has been proposed to estimate the casing wear in curved sections of the wellbore that have the drill string lying on its low side. The fundamental assumption of this model is that the volume worn away from the casing wall is proportional to the work done by friction on its inner wall by the tool joints only. It also assumes that the primary mechanism for casing wear is the rotation of the drill string, and the wear caused during tripping is insignificant. The borehole torsion models of wellbore trajectory, namely spatial-arc, natural-curve, cylindrical-helix, and constant-tool face, have been incorporated separately to enhance the accuracy of estimating the wear volume downhole. The wear-energy model for a detailed analysis of a practical example using real-time well survey data will be presented. Wear zones along the wellbore have been identified using a mathematical criterion of the “contact zone parameter.” The wear-groove depths for each contact zone along with an equivalent average wear for the curved section of the wellbore have been estimated. The wear volumes predicted by the various curvature and torsion models of wellbore energy have been graphically studied. The wellbore torsion has been found to significantly impact the casing-wear downhole.

Analysis of Uncertainty Attributed to Measurement Error in Directional Surveying With Different Models

The current interest in directional survey calculations is related to the increased number of highly deviated holes drilled from offshore platforms. Positional accuracy is important because, when two wellbores closely approach one another, it is essential to avoid intersection. Also, when a relief well is drilled, it is important to achieve intersection with the wild cat well. Unfortunately, there is no set of calculations that can be used to analytically determine exact bottomhole position relative to the top hole. Therefore, uncertainty with respect to estimating the wellbore position should be determined using directional surveys and the different models being used.Errors in estimating the target primarily derive from two sources — model errors and measurement errors. Model errors occur in the calculations used to estimate borehole position because of a lack of complete information, which can only be tackled by having an infinite number of survey stations throughout the well path. On the contrary, measurement errors occur in the readings of inclination angle (I), azimuthal angle (A), and the distance between two stations (L). This paper focuses on estimation of measurement error, which is an inherent property of measuring instruments being used for the survey and can be evaluated numerically after making some assumptions necessary for such an analysis. The said analysis would ultimately provide ellipses of uncertainty for the target point based on different survey models.The study is based on a paper by Walstrom et al. [1], which uses the angle averaging (AA) method for survey calculations. The study is extended for calculating measurement errors in the following models: the radius of curvature (ROC), minimum curvature (MC), and natural curve (NC) methods.Copyright