Hole Completion Research Articles

Analysis of thermal production effect of multi-horizontal U-shaped well

The multi-horizontal section hot dry rock (HDR) heat extraction system is proposed for efficient deep geothermal resources mining in this study. The thermal-hydraulic numerical model of the multi-horizontal section geothermal exploitation system is set up, the space-time evolution of temperature field is analyzed. The sensitivity factors including completion schemes, horizontal well diameter, quantity of horizontal sections and spacing of horizontal sections are investigated. The simulation results show that the production temperature and heating power of the multi-horizontal HDR production system decrease with the development of mining, and the decreasing speed gradually slows down. The open hole completion can be preferred when the formation conditions permit. The variation of horizontal well diameter has little effect on heat production when other conditions remain unchanged. The quantity of horizontal sections to be 3 or 4 for better heat extraction performance under the spacing of horizontal section remains 300m and the spacing of the horizontal section to be 150m is recommended within the scoped of this study. The results of this study can provide guidance for HDR mining research.

Evaluation of completing horizontal oil wells with inflow control devices

Oil production in horizontal wells currently has challenges such as excessive water production, which reduces the cumulative oil recovery, this occurs due to water coning in the reservoir which resulted to earlier water breakthrough in the wellbore. Various Down hole Flow control Devices (DFCs) including ICDs, AICDs, and AICVS are typically installed in the horizontal section of oil wells to mitigate the problem. The study of autonomous ICDs was done by comparing the long term flow performance of such devices against passive ICDs. The study was carried out with a reservoir model representing one horizontal well with three completion alternatives: open hole, ICD, and AICDs which were perforating in numerical reservoir model, these cases were run in the Eclipse simulator and their results were compared with the open hole. The results showed that the installation of Autonomous ICDs (AICDs) allowed producing more oil when compared against using passive ICDs. As it was found in three cases, for well X2, production with AICD increased the overall oil recovery by 257% and reduction of water cut to 42% as compared against an open hole completion, whereas production with ICD increased the overall oil recovery by 192% and reduction of water cut to 50% as compared against an open hole completion. The results proved that the installation of ICDs increased the net revenue comparing to open hole completion.

Experimental Investigation of Fracture Propagation in Clayey Silt Hydrate-Bearing Sediments

More than 90% of the natural gas hydrate resources are reserved as marine clayey silt sediments. It is of great significance to efficiently develop a clayey silt hydrate. At present, there are problems of low single well production and small depressurization range in its production test, which is still a long way from commercial exploitation. The combination of hydraulic fracturing technology and other methods such as depressurization method is regarded as one of the potential technical means to achieve the commercial exploitation of the hydrate. However, compared with shale gas reservoirs and coalbed methane reservoirs, clayey silt hydrate reservoirs have special mechanical properties, resulting in unique hydraulic fracturing processes. Therefore, it is necessary to study the fracture initiation and propagation laws of clayey silt hydrate reservoirs. To this end, we carried out large-scale (30 × 30 × 30 cm) true triaxial hydraulic fracturing experiments using a simulated material with similar mechanics, porosity, and permeability to clayey silt hydrate-bearing sediments. The effects of completion method, fracturing method, and fracturing fluid displacement on hydraulic fracture propagation of clayey silt hydrate-bearing sediments were studied. The results showed that a perforated completion can significantly increase the fracture reconstruction area and decrease the fracture initiation pressure compared to an open hole completion. Due to the small horizontal stress difference, it is feasible to carry out temporary plugging fracturing in clayey silt hydrate reservoirs. Temporary plugging fracturing can form steering fractures and significantly improve fracture complexity and fracture area. Increasing the fracturing fluid displacement can significantly increase the fracture area as well. When conducting fracturing in clayey silt hydrate-bearing sediments, the fracturing fluid filtration area is obviously larger than the fracture propagation area. Therefore, it is recommended to use a high-viscosity fracturing fluid to reduce the filtration of the fracturing fluid and improve the fracturing fluid efficiency. This study preliminarily explores the feasibility of temporary plugging fracturing in clayey silt hydrate reservoirs and analyzes the effect of completion methods on the propagation of fracturing fractures, which can provide a reference for the research conducted on the fracturing stimulation of clayey silt hydrate reservoirs.

In horizon penetrating coordinates: Kerr black hole metric perturbation, construction and completion

We investigate the Teukolsky equation in horizon-penetrating coordinates to study the behavior of perturbation waves crossing the outer horizon. For this purpose, we use the null ingoing/outgoing Eddington–Finkelstein coordinates. The first derivative of the radial equation is a Fuchsian differential equation with an additional regular singularity to the ones the radial one has. The radial functions satisfy the physical boundary conditions without imposing any regularity conditions. We also observe that the Hertz-Weyl scalar equations preserve their angular and radial signatures in these coordinates. Using the angular equation, we construct the metric perturbation for a circularly orbiting perturber around a black hole in Kerr spacetime in a horizon-penetrating setting. Furthermore, we completed the missing metric pieces due to the mass M and angular momentum J perturbations. We also provide an explicit formula for the metric perturbation as a function of the radial part, its derivative, and the angular part of the solution to the Teukolsky equation. Finally, we discuss the importance of the extra singularity in the radial derivative for the convergence of the metric expansion.

Image completion based on segmentation using neutrosophic sets

Image completion aims to restore the corrupted regions in an image. One of the most important challenges in image completion is to find the most appropriate data for replacing in the hole. In this paper, we proposed a novel image completion method which applies neutrosophic-based segmentation to fill in the hole. Since neutrosophic is useful to interpret indeterminacy included in images, we applied neutrosophic-based image segmentation to decrease spatial and intensity ambiguities exist in images while we have more boundary and homogeneity preservation and less discontinuity. Our exemplar-based image completion algorithm starts from the outer pixel with the maximum priority and iterates until there is not any pixel in target region. Our extended similarity measure considers both neighbourhood and similarity based on results of our innovative neutrosophic-based image segmentation algorithm to find the patches with the maximum match for hole completion. Results show that our approach introduces an improvement of 18% for ASVS (Average Squared Visual Salience) compared to earlier methods. We also gained 0.9919 and 38.96 for MSSIM (Mean of Structure Similarity) and PSNR (Peak Signal to Noise Ratio), respectively while the best values for earlier methods were 0.9868 and 36.75 for MSSIM and PSNR, respectively which is the effect of using neutrosophic segmentation.

Point Cloud Scene Completion with Joint Color and Semantic Estimation from Single RGB-D Image.

We present a deep reinforcement learning method of progressive view inpainting for colored semantic point cloud scene completion under volume guidance, achieving high-quality scene reconstruction from only a single RGB-D image with severe occlusion. Our approach is end-to-end, consisting of three modules: 3D scene volume reconstruction, 2D RGB-D and segmentation image inpainting, and multi-view selection for completion. Given a single RGB-D image, our method first predicts its semantic segmentation map and goes through the 3D volume branch to obtain a volumetric scene reconstruction as a guide to the next view inpainting step, which attempts to make up the missing information; the third step involves projecting the volume under the same view of the input, concatenating them to complete the current view RGB-D and segmentation map, and integrating all RGB-D and segmentation maps into the point cloud. Since the occluded areas are unavailable, we resort to a A3C network to glance around and pick the next best view for large hole completion progressively until a scene is adequately reconstructed while guaranteeing validity. All steps are learned jointly to achieve robust and consistent results. We perform qualitative and quantitative evaluations with extensive experiments on the 3D-FUTURE data, obtaining better results than state-of-the-arts.

Self-adaptive micro-hole breakout detection in the electrochemical discharge drilling process based on CNN-BiLSTM

Electrochemical discharge machining (ECDM) is an effective and promising technology for the micro-hole drilling of hard and brittle insulating materials such as glass, quartz and ceramics. Hole exit quality is an important part of through-hole machining quality in ECDM. The hole exit quality is influenced by two different machining states before and after the hole breakout. Therefore, it is necessary to identify hole breakout occurrence to provide a basis for achieving a machining strategy that matches the changing machining state and thus improves the hole exit quality. However, currently, there is no effective method to detect the hole breakout in the ECDM of micro-holes. In this paper, a self-adaptive micro-hole breakout detection method based on CNN- BiLSTM was proposed. The hole breakout detection can be expressed as a binary classification problem according to the analysis of the current signals collected before and after the hole breakout. The sufficiency of the raw current signal information was discussed. A CNN-BiLSTM model was established to detect hole breakout using the current signals as the training dataset. The CNN-BiLSTM model showed better performance compared to CNN, LSTM, BiLSTM and CNN-LSTM by comprehensive considering of accuracy, F1 score, AUC value, training time, predict time, breakout detection results and Dunn test results. Furthermore, the length of current signals used for each single detection and the threshold value used for breakout detection were discussed. An optimal threshold value was calculated, and the delay of detection was within 66 ms. The proposed method was successfully applied to the hole breakout detection in the through-hole drilling experiments and the success rate was 100 %. The hole exit area just after breakout were measured and compared to the hole exit area after hole completion, the average ratios of the former to the latter were 11.69 %, 27.29 %, 38.43 % and 44.41 % for the voltage of 39 V, 41 V, 43 V and 45 V respectively.

An economic-energetic-environmental evaluation algorithm for hybrid mid-depth geothermal heating system

Low-carbon and high-efficiency mid-depth ground-source heat pumps become a promising heating option for cold regions in China towards carbon neutral. However, system-level evaluation algorithm that incorporates multiple influences are needed. Hence, an economic-energetic-environmental evaluation algorithm for a hybrid mid-depth geothermal heating system was established. It couples load characteristics and reflects the impact of design parameters, geological conditions, and fluctuating factors on three evaluation indicators. The economic performance increases initially as heat pump load ratio and hole depth grow but is more sensitive to the former. A recommended design parameters’ zone is given for the general case. Moreover, when the load ratio is 0.65 and the depth is 2750 m, the maximum relative NPV reaches 88.4 million CNY. The peak value of carbon reduction revenue, 54.7 CNY/tCO2, appears near the load ratio of 0.63 and the depth of 3300 m. The valley of heating cost, −15.9 CNY/GJ, occurs near the load ratio of 0.66 and the depth of 2600 m. The break-even point of geothermal gradient has a positive linear relationship with the hole completion cost, by which investing feasibility can be determined. Fluctuating factors have a significant impact on the above indicators but will not change the profit-loss status.

Prediction of Sanding Likelihood Intervals Using Different Approaches

Sand production is undesirable matters, occurring in wells that are producing from sand reservoirs. It causes many problems such as erosion and grains accumulation in downhole and surface equipment’s, and formation subsidence. Important stage in sanding problem solution is a prediction of likelihood sand production intervals. In present paper, a vertical well X1 that is producing from Asmari reservoir in Y field at southern Iraq was selected for study. Asmari reservoir was classified to six units: A, B1, B2, B3, B4, and C. B zones consisted from sandstone with others rock types. Eight approaches were used for prediction sanding onset intervals by dealing with X1 well as open hole completion. Utilized eight prediction methods are compressional sonic wave (CSW), unconfined compressive strength (UCS), total porosity (PHIT), shear modulus to bulk compressibility (G/Cb), B-Index, Schlumberger index (S- Index), combined index (Ec-Index) and critical drawdown pressure (CDDP). All these methods performed based on 2462 measured points of CSW, sonic shear wave log (SSW), and density log (DL). Sand production likelihood intervals was selected by determination of cutoff values of adopted methods. Sand is possible to occur if interval has values lower than cutoff values of G/Cb, UCS, B-Index, S-Index, Ec, and CDDP and greater than cutoff values of CSW, and PHIT. Obtained cutoff values of eight approaches were 800 x 109 psi2, 36 Mpa, 0.2, 80 us/ft, 10000 Mpa, 108 Mpa, and 2700 Mpa, of G/Cb, UCS, PHIT, CSW, B-Index, S-Index, and Ec respectively. As well as sand production is possible to occur of bottomhole flowing pressure lower than calculated CDDP. Some Intervals had high CDDP that referred to abnormal pressure zones consisted from shale. Determination of sand onset intervals is a key for selecting best methods for controlling.

A Productivity Prediction Model for Multistage Fractured Horizontal Wells in Shale Gas Reservoirs

The tiny sizes of pores and throats in shale gas reservoir increase the complexities of the flow mechanisms, which challenge the accuracy of current productivity models during optimization design and prediction for multistage fractured horizontal wells (MFHWs). This paper established a productivity prediction model for MFHWs with cased hole completion and open hole completion. The model couples the gas flow in the matrix, fractures, and wellbore using potential superposition principle and considers multiple mechanisms of gas flows in a shale gas reservoir, such as stress-sensitive effect, wellbore friction, as well as interference between fractures. Based on the developed model, a productivity prediction method for MFHWs in shale gas reservoirs has been established, and sensitivity of the impact factors on productivity was analyzed. The analysis revealed that MFHWs are suitable for shale gas reservoirs with certain thickness. The number of fractures has significant impact on the production of shale gas reservoir. And the fractures at the both sides have higher production rate than the intermediate ones for multistage fractured horizontal wells. The fracture half-length has insignificant effect on the productivity of MFHWs. The production of shale gas reservoir also increases with pressure drop. The wellbore friction has insignificant effect on the production of MFHWs. The findings of this study can provide a basis for scientific evaluation of the production of MFHWs in shale gas reservoirs.

Evaluating the screen performance in sand control design by laboratory sand retention test

To produce oil and gas from unconsolidated sandstone reservoirs, sand screens are necessary to control sand flowing from the formation into the wells, causing well pluggings. In open hole completion, stand-alone sand screen is an essential component to prevent sand production, thus, selection of suitable sand screens is critical to minimize sand production and optimize the well’s production life. 
 Nowadays, premium screens with steel meshes make the openings of the screen slots complex in shape, resulting in a very different effect of sand retention compared to the traditional screens. The sand retention test is considered a standard and must be conducted to select screen opening and size, which decides the sand prevention capacity of the screen. 
 The paper presents the factors affecting the results of laboratory sand retention tests and proposes solution in the direction of considering the data trend rather than relying completely on the output results. The paper also brings about the evaluation method through the change the screen’s permeability to have better qualitative results compared to the traditional methods.

Modelling the near-wellbore rock fracture tortuosity: Role of casing-cement-rock well system, perforation and in-situ stress

Near-wellbore rock fracture is a key subject in subsurface energy extraction. Casing perforation completion is perhaps the most used type of well design mainly in comparison to the open hole completion. However, the effects of the combined casing-cementing-rock structure on pressure transmission in fracturing the rock have not been sufficiently addressed, but is key to understanding some important phenomena in hydraulic fracturing, such as fracture tortuosity. Here we develop a combined analytical-numerical model to investigate the rock fracture mechanism near the wellbore with realistic consideration of the boundary condition imposed by the complete well system. Different well configurations can lead to variation in the pressure applied on the rock formation and hence affect the fracture propagation near the wellbore. Our results show, as the pressure transmission through the well structure is enhanced, the fracture propagates more closely to the perforation orientation with a smaller deflection angle and the breakdown pressure is increased. The near-well tortuosity of the crack can be reduced by decreasing the thickness of casing and cement sheath, reducing Young's modulus of casing and increasing Young's modulus of cement up to around 15 GPa, based on the realistic range of values of the well parameters. The effects of in-situ stress condition and perforation angle and length on the near-well cracking are also investigated. The developed model can be used to aid decision making in terms of improved understanding of hydraulic fracturing technology and well design optimization.

Maximizing well productivity by using filter cake breaker for synthetic-based mud drill-in fluid (SBMDIF) system

• Formation damage has a significant impact on the overall performance. • An effective filter cake breaker selection is critical to minimize formation damage. • The straightforward reaction of meso‑surfactant and nano-surfactant filter cake breakers with barite. • Surfactant-based filter cake breaker was more efficient in removing synthetic-based drilling mud. Formation damage has a significant impact on the overall performance of the well productivity. Removal of filter cake which is deemed to be the main strategy of formation damage remediation is crucial in open hole completion with pre-slotted liner or stand-alone screen (SAS) as a mean of sand control. Without proper planning, inefficient filter cake removal can lead to tremendous consequences since filter cake can plug the sand control component. Making the condition worse, sand control component is susceptible to plugging. This highlights the importance of selecting an effective filter cake breaker that can successfully remove the filter cake through dissolution of the main solids that constitute the major portions of the filter cake which could be the weighting material, barite, or calcium carbonate. Besides that, a proper understanding of the mechanism of the filter cake breaker chemical would be very beneficial to comprehend the filter cake breaker efficiency. The laboratory study attempted to emulate the reservoir condition. Regained permeability testing using High Pressure High Temperature (HPHT) Filter Press aimed to test the ability of several commercial filter cake breakers in removing synthetic-based-mud drill-in-fluids (SBMDIF). Chelating-based filter cake breaker, meso‑surfactant-based filter cake breaker and nano-surfactant-based filter cake breaker were the samples to be tested in the laboratory work. The condition of the filter cake after being soaked statically was visually interpreted and the regain permeability was recorded. The mechanism of each filter cake breaker to remove the SBMDIF filter cake was also examined. Based on the experimental study, meso‑surfactant-based filter cake breaker was found to be more effective to remove SBMDIF filter cake compared to chelating-based filter cake breaker and nano-surfactant-based filter cake breaker.

Automated measurement of dairy cows body size via 3D point cloud data analysis

Body measurement plays an important role in the breeding and production of dairy cows. Body measurement is mainly done manually, which is laborious, imprecise, and prone to stressful behavior in animals. A portable non-contact 3D measurement system of dairy cow body using smartphones was proposed in this study. Reconstruction of 3D point clouds scene containing the cow through a Structure-from-Motion (SfM) photogrammetry, the RANdom SAmple Consensus (RANSAC), and the Euclidean clustering were used to segment the scene and extract cow’s point clouds. Aiming to solve the holes in the cow point cloud caused by the barrier and legs occlusion, a completion method based on the spline curve with a smoothing factor was proposed. The automatic measurement of body size was realized through morphological features. The accuracy of the proposed system was evaluated in a commercial dairy farm. Compared with mesh-based method, the proposed point cloud hole completion method reduced the maximum relative errors of chest girth and chest width by 2.22% and 2.18%. Under different cow postures, the average relative error between automatic body size measurement algorithm and manual calibration was less than 4.67%. The average relative error between the measured value of the non-contact measurement system and the actual value of withers height, body length, chest girth, and chest width were 2.41%, 3.18%, 4.37%, and 6.12%, respectively. The results showed that the proposed method can be used as an automatic and non-contact approach for measuring animal body size.

Technique Models Effect of Local Doglegs on Running of Completions and Casing in Hole

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 202431, “RSS and Motor Directional Analysis: Modeling the Effect of Local Doglegs on Running in Hole of Completions and Casing,” by Benjamin Nobbs, SPE, Stéphane Menand, SPE, and Ngoc-Ha Dao, SPE, DrillScan, prepared for the 2020 SPE Asia Pacific Oil and Gas Conference and Exhibition, originally scheduled to be held in Perth, Australia, 20–22 October. The paper has not been peer reviewed. Rotary steerable systems (RSS) and steerable motors pose unique challenges when modeling bottomhole-assembly (BHA) directional behavior. The complete paper presents a methodology that aims to allow the anticipation of problems such as mechanically stuck pipe or lockup situations when running in hole (RIH) casing or completion strings. Correctly evaluating the risk of BHA, casing, or completion strings being stuck or locked up when RIH can ultimately provide a template for ultimate reduction of nonproductive time. Local Doglegs and Downhole Equipment A pure rotary BHA has no ability to change its directional behavior during the run. Its directional capabilities are defined by stabilizer position and outer diameter, and, although no ability exists to control directional behavior directly, this type of BHA may be sensitive to weight on bit (WOB), mud weight, formation changes [unconfined compressive strength (UCS)], and inclination. With proper design, it is possible to design a rotary BHA that is insensitive to these parameters. A motor and turbine can operate with or without a bent housing. A bent housing imposes a small angle offset from the axis of the drillstring, with the result that two distinct modes of directional behavior occur, sliding and rotating. Because of its increased rotary speed, rotary mode is prioritized over sliding mode (ideally, 30% sliding and 70% rotating). Because a combination of these distinct drilling modes exists, the actual tortuosity may be quite different when compared with standard survey spacing (Fig. 1). A slight overgauge will be present when operating in rotating mode because of the bent sub. Much like a rotary BHA, a motor may be sensitive to operational parameters. The type of polycrystalline diamond compact bit recommended is one with a long passive gauge that is required to drill in the direction of bit tilt. An RSS can be modelled in two distinctive modes: point-the-bit and push-the-bit. A point-the-bit system operates in much the same way as a downhole motor by varying the tilt at bit. A push-the-bit system operates by applying a side force at pads that pushes against the wellbore. Both types of RSS allow continuous rotation of the drillstring and reduce the risk of differential sticking and, ultimately, stuck pipe. A point-the-bit system may operate with or without proportional steering. A push-the-bit RSS system can operate either by varying the magnitude of force or having a constant force that is varied for a set period in the desired tool-face orientation (TFO). Because of the method of activation (using pads that hit against the side of the wellbore), the local doglegs that can be induced are slightly higher.

Acute Physiological Effects of Continuous Versus Intermittent Walking During Golf in Individuals With Knee Osteoarthritis: A Pilot Study.

The aim of the study was to compare the acute effects of walking the golf course versus using a golf cart during a round of golf on biological markers of joint disease, joint pain, and cardiovascular parameters in individuals with knee osteoarthritis. Participants with knee OA (n = 10) older than 50 yrs were recruited for this crossover designed study in which they completed two 18-hole rounds of golf: (1) walking the course and (2) using a golf cart. Five control participants (n = 5) performed the walking condition only. Step count, heart rate, rating of perceived exertion and pain using the Numeric Pain Rating Scale were measured during the round. Serum was collected at baseline, 9th hole (halfway), and 18th hole (completion) and tested for biomarkers associated with tissue turnover (cartilage oligomeric matrix protein), inflammation (tumor necrosis factor α, interleukin 1β, interleukin 6), and degradative enzyme production (matrix metalloproteinase 3, matrix metalloproteinase 13). In knee OA participants, walking the course was associated with significantly higher step count and duration of moderate/vigorous physical activity (72.2% vs. 32.6% of the round) but did lead to a significant increase in knee joint pain (P < 0.05). Both conditions caused cartilage oligomeric matrix protein and matrix metalloproteinase 13 concentration increases from baseline to completion (P < 0.05), but inflammatory markers (tumor necrosis factor α, interleukin 6, and interleukin 1β, P < 0.05) only increased when walking the course. Biomarker concentrations did not increase in control participants. Walking the course optimizes the duration of moderate/vigorous activity during a round of golf, but the golf cart is a beneficial option in those with exacerbated joint pain and inflammation that would otherwise limit participation.

A multi-objective optimization method of inflow control device configuration

Inflow control device (ICD) restricts flows of certain sections to achieve a uniform push of water oil contact along a horizontal well. The efficiency of ICD depends on its configuration (location, number of nozzles, and nozzle sizes). Traditionally, optimal ICD configurations have been selected using a trial and error approach simulating different configurations. This method is time-consuming and the outcome can be limited by pre-set candidates, thus may result in non-optimal configurations. In this paper, a method involving several optimization objectives is proposed to obtain the optimal ICD configuration, in order to reach an optimal total production before water breakthrough. A flow coupling model (permeation flow – nozzle flow – inner tubing flow) of horizontal wells completed by ICDs is established to calculate the inflow profile and the pressure distribution along the horizontal wellbore. The production rate and the water breakthrough time are the optimization objectives, and the sand control condition (production pressure drawdown < critical pressure drawdown of sanding onset) is taken as one of the constraints of optimization. Then the NSGA-II algorithm is employed to search for the Pareto frontier based on the model of coupled flow. Selected by the TOPSIS method from the Pareto frontier, the optimal ICD configuration ensuring optimal total production and no sanding is acquired. The original data of well A1 in SL oilfield are used for case study. The optimal configuration is compared with single-objective optimization results and open hole completion in terms of production rate and water breakthrough time. By predicting the total productions of different configurations before water breakthrough, we think the optimization method is capable of determining the optimal configuration. This research can provide some theoretical support for the determination of ICD configuration in horizontal wells.

Rock mechanics and wellbore stability of deep shale during drilling and completion processes

The seepage coupling effect between the borehole rock mass and borehole fluid and the effective stress field distribution around the wellbore are obviously different under different working conditions of drilling, fracturing and completion, which affect the stability of horizontal borehole of shale gas reservoir. The physical, chemical, and mechanical properties of the shale in Longmaxi formations in a block of China immersed with different working fluids were tested experimentally. Combined with the experimental data, a theoretical model was established to evaluate and analyze the wellbore stability of Longmaxi formations shale under different working conditions of drilling, fracturing and completion. The results show that the shale of the Longmaxi Formation is a typical hard and brittle shale, the bedrock is dense, with high mechanical strength and weak hydration and expansion ability. The immersion effect of different working fluids has little influence on the expansion and mechanical properties of shale bedrock. However, the mechanical strength of shale with relatively developed bedding fractures is low, and the immersion effect of different drilling fluid further weakens the mechanical strength, resulting in obvious anisotropy of mechanical strength of underground rock. The coupling effect of seepage between wellbore and formation has obvious influence on the dynamic distribution of borehole pore pressure and wellbore stability under different working conditions. The effective fluid column pressure at the bottom of the hole is the highest during the fracturing process, which leads to the increase of pore pressure near the wellbore, and then to the increase of collapse pressure around the borehole wall. The equivalent density of collapse pressure rises to the maximum of 1.93 g/cm3, and the wellbore stability is the worst in the fracturing process. The wellbore stability is secondary in the drilling process, and the wellbore stability is good when drilling along the direction of the maximum horizontal principal stress, and the equivalent density of collapse pressure is 1.69 g/cm3. During the completion process, the formation fluid flows into the wellbore, the pore pressure in the borehole is relieved, and the equivalent density of the collapse pressure is reduced to approximately 1.35 g/cm3, the wellbore stability is the best and open hole completion can be attempted in the horizontal well section. The results provide a certain theoretical reference for the selection of the drilling and completion schemes in shale formation.

Completion detection and efficiency improvement for breakout stage of fast EDM drilling

Electrical discharge machining (EDM) is a non-conventional machining process that removes material by consecutive electrical sparks. Fast EDM drilling is one of such processes, which employs rotary tubular electrodes and inner flushing to enhance the machining efficiency and stability. It is widely used to produce small holes on various kinds of products. The breakout event occurs when the electrode just penetrates the workpiece during the drilling process, which marks the start of the breakout stage of the process. At this moment, the outlet of the hole is smaller than the desired due to severe radial tool wear, and the hole is not completed. As a result, further feeding is needed before the electrode fully penetrates the workpiece. To avoid back-strikes, the hole completion detection is crucial. However, the uncertainty of longitudinal and radial tool wear makes it difficult. This paper proposes a novel method to detect the hole completion by analyzing the discharge signals in the breakout stage. Besides, for the machining efficiency to become much lower in the breakout stage, the control strategy is also proposed to improve it by adjusting the reference voltage and gain factor of the servo controller. The optimal parameters were found by a full-factorial experiment. Experiments were conducted to validate the proposed methods by drilling holes at various inclinations. The results show that the proposed hole completion detection method could successfully sense the hole completion without misjudgments. And the electrode was fed at most 1.81 mm out of the hole outlet when the detection was made. Besides, the proposed control strategy improved the machining efficiency by up to 56.2% in the breakout stage in the experiments.

Simulation of wellbore erosion and sand transport in long horizontal wells producing gas at high velocities

Horizontal wells play a crucial role in improving the recovery of hydrocarbons from thin reservoirs. In weakly consolidated formations, these wells may encounter sand production and this may lead to several problems including wellbore instability, production time loss, corrosion and plugging of downhole and surface facilities. Sand production in wellbores can be linked to two physical phenomena. The first is stress concentration along the wellbore causing mechanical instability and localized failure of the rock. The second is related to hydro-mechanical forces that cause internal and surface erosion, triggering the release of particles from rock surface, by the action of drag forces of produced matter. In open hole completions, high-velocity gas flow can trigger the release of particles from rock surface.We present a reservoir-wellbore flow model to simulate high-velocity gas flow in porous media and in open-hole horizontal wells. Since gas wells are associated with high-velocity flows, this study is focused on dry gas production from long open-hole wells, where large sections of wellbore wall are exposed to the stresses caused by the flowing stream of gas. The Forchheimer equation is coupled with conservation of mass to solve for gas flow in the porous media while the Bernoulli's equation is coupled with mass conservation to solve for gas flow in the horizontal wellbore. Computed wellbore velocities are then used to calculate sand erosion using the Water Erosion Prediction Project (WEPP) model. Subsequently, an advection equation is used to model the transport of the eroded sand particles in the wellbore. An example is presented to study how wellbore erosion affects the flow of gas and pressure drop in the horizontal well. The results indicate that the erosion due to high-velocity gas flow in open-hole horizontal wells affects the gas production rate as well as the well flowing pressure.