Irregular Borehole Research Articles

The stress relief correction method considering irregular borehole shapes

In situ stress is an essential parameter that directly affects all engineering construction and scientific research in deep rock masses. The shapes of deep borehole are often highly irregular, so that the traditional in situ stress measurement methods based on the assumption of circular or cylindrical boreholes are no longer applicable. We propose a stress relief correction method considering irregular borehole shape using the complex function method of elasticity. The conformal mapping function is introduced to project the irregular shape into a unit circle and the relationship between the strains at the borehole wall and the far-field in situ stress components are solved by the Cauchy integral methods. The results show the following: (1) The stress results from the stress relief correction method are consistent with the theoretical solutions and numerical simulation results for circular and square boreholes. (2) The irregular borehole shape significantly changes the distribution of stress magnitude and the direction of strain measurement on the borehole wall. The stress relief correction method considering irregular borehole shapes provides an essential theoretical basis for deep rock mass engineering construction and deep oil and gas field exploration.

Dynamic Analysis of Downhole Tool String in Uplift and Downshoot Motion

Abstract Running various downhole tools in extended reach wells is usually very important for well operations in the oil production engineering. The various bottlenecks due to high drags or irregular boreholes are always the main reasons for tool resistance. Failure to reach target depth often has a profound consequence on well deliverability and overall cost. In this paper, we proposed an “uplift and downshoot” method to run the tool string. In this method, the tool may first be raised to a certain height (raising process), and then lowered quickly so that the tool “falls” from this height at a relatively fast velocity to help rush through the stuck points. The soft string model considering the effects of lowering velocity is established to simulate the tool falling process. By solving the tool axial acceleration in each falling interval with energy conservation, the axial force and acceleration distributed in the tool string during the entire running process can be determined. A case study of lowering measuring instruments in the completion of tubular of a water injection well is analyzed by the proposed method. In the assumed two situations with and without stuck points, the tool is lifted 15 m to simulate the rush through the process. Velocity and acceleration of every lowing height under different stuck resistances are drawn and listed to judge the tool passability. This work provides a way to run downhole tools and a matched theoretical method to help field operators lower downhole tools efficiently.

Data-driven construction of Three-dimensional subsurface geological models from limited Site-specific boreholes and prior geological knowledge for underground digital twin

A digital twin is a digital replica or virtual representations of 3D physical entities in the real world. In practice, it is challenging for 3D modelling of subsurface stratigraphy in an underground digital twin due to insufficient site-specific measurements and a lack of efficient 3D spatial prediction tools. In this study, a data-driven and deep learning method, called IC-XGBoost3D, is proposed to build a 3D geological model from limited site-specific boreholes and 2D training images reflecting prior geological knowledge. Anisotropic stratigraphic relationships are learned from two perpendicular 2D training images, and the extracted stratigraphic statistics serve as the input for pre-training a 2D simulation slice. A sequence of 2D simulation slices is then simulated with constraints by site-specific boreholes and subsequently reassembled into a 3D geological model. Note that the parameters of the deep learning method are calibrated with site-specific data and prior training images before being applied to develop the geological model. The model performance is demonstrated and validated using an illustrative example. The proposed method can efficiently generate an anisotropic 3D geological model as a point cloud from two perpendicular training images and sparse boreholes with a high prediction accuracy. More importantly, the proposed method not only infers the most probable 3D geological domain, but also provides a quantitative evaluation of associated 3D stratigraphic uncertainty. Effects of irregular borehole spacing and single training image on the simulation performance are also investigated.

3-D Numerical Mode Matching Method for Off- Centered Electromagnetic Well Logging Tools in Noncircular Vertical Borehole and Invasion Zones in Multilayered Media

In electromagnetic (EM) well logging for petroleum exploration, off-centered tools within a noncircular borehole and invasion zones in multilayered media represent a challenging 3-D problem for traditional numerical methods. The 3-D finite-element numerical mode-matching (FNMM) method and the spectral numerical mode-matching (SNMM) method are developed to address this problem in this work. The numerical mode-matching (NMM) methods reduce the original 3-D well logging problem into a series of 2-D open waveguide eigenvalue problems plus a 1-D layered medium problem, which can be analytically solved by a recursion procedure. These waveguide eigenvalue problems with anisotropic inhomogeneous media in the horizontal directions are solved numerically by the mixed finite-element method (MFEM) with the flexibility for the complex geometry and the mixed spectral element method (MSEM) with the exponential convergence. A general operator form of Maxwell’s equations is also derived to obtain the source excitation vector independent of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$z$ </tex-math></inline-formula> -axis. Therefore, the NMM solvers can effectively and accurately simulate the response of off-centered EM well logging tools in an irregular borehole in complex formations. The NMM methods are applied to simulate three formations including a five-layer isotropic homogeneous formation, an off-centered logging tool in a noncircular borehole with anisotropic invasion zones in a five-layer anisotropic formation, and the Oklahoma formation. Numerical experiments indicate that the NMM methods are highly efficient and accurate for the EM well logging models.

A new analysis model for borehole stability in gas drilling

Gas drilling is one of the main underbalanced drilling techniques, which has a significant effect on the ROP (rate of penetration) improvement in hard formation. However, because the circulating medium in drilling process is gas instead of liquid, the fluid column pressure of gas drilling cannot provide enough support force for the rock around borehole as that of conventional overbalanced drilling. Due to the combination influence of engineering and geological factors, the borehole rock is prone to instability during gas drilling, which can lead to rock blocks falling from the well wall and the formation of irregular borehole shape. To avoid the occurrence of complex underground accidents caused by too many falling rock blocks, such as the borehole instability and pipe-sticking, it is necessary for gas drilling to take into consideration the true triaxial stress state of the rock in the deep formation and the possibility of the borehole breakout. Based on the permitted breakout angle of borehole and true triaxial strength theory of rock, an analysis model of borehole stability has been developed for the engineering of gas drilling. The theory of linear elasticity, involved in this analysis model, is used to analyse the stress state of rock around a cylindrical hole under lower wellbore pressure. The failure of the rock around borehole is determined by the Mogi-Coulomb criterion, which considers the intermediate principal stress effect on rock material. The results show that the model is as simple and practical as the traditional model, which takes no account the effect of the intermediate principal stress or the permitted borehole breakout, and it can provide an accurate analysis for the borehole stability of gas drilling. Therefore, the results of this study, combined with the related work such as the prediction of water formation, can guide the adaptability evaluation for the engineering design of gas drilling.

Design and construction of test facility for evaluation of swell packers in cased and open holes

In the petroleum industry, packers refer to the components/products which are used to isolate one section in a formation from others, or to isolate the outer section of a production tubing from the inner section, which may be a casing or a liner or the wellbore itself. Mechanical packers are installed through some form of tubing movement. Permanent packers are low in cost and provide better seal. Retrievable packers have lower sealing capability but are re-usable after repair, and therefore expensive. More innovative packers made of swellable elastomers are rather new. These elastomers swell when they come in contact with different types of fluids (mostly water and oil). Like a casing, these new packers are lowered to the desired depth and allowed to swell before production or injection operations begin. Various studies have been published about the swelling and mechanical response of these elastomers. However, many questions remain about the sealing performance of swell packers, and their effectiveness in actual wellbores of rough and random profiles. This paper describes the design and development of an experimental unit for performance testing of a mix of inert and swelling elastomer seals under realistic well conditions. Actual swell packers and petroleum tubulars are housed in a concrete block that replicates open-hole and cased-hole wells through layers of varying roughness. This is a one-of-its-kind unit providing full-scale demonstration about the working of swell packers against outer casing or rock formation in a wellbore. It is also a complete testing apparatus to investigate the behavior of swell packers made of fast-swell and medium-swell elastomers, and how they seal off irregular boreholes. This work can provide basic understanding and visual corroboration to petroleum engineers and students, swelling elastomer application developers, and academic and research personnel.

Research on Calculation Model of Horizontal Wellbore Shape with Non-uniform In-situ Stress

In horizontal Wells, non-uniform ground stress is the root cause of irregular borehole shape. The failure of caliper instrument in horizontal section leads to the failure of accurate evaluation of borehole shape, which makes it difficult to calculate the frictional resistance of tubing string in the later stage. This paper studies the borehole shape calculation model from the following two aspects: (1) in the aspect of diameter expansion caused by shaft wall collapse. By establishing coulomb-Mohr criterion in cylindrical coordinate system, the well wall collapse instability is judged, and the hole diameter expansion is evaluated by establishing the calculation model of the instability range. (2) Shrinkage caused by stress concentration. Firstly, the boundary position of formation deformation is determined. Secondly, the calculation model of borehole deformation is established by analyzing the stress characteristics of formation, which reflects the time effect of borehole deformation. Therefore, the establishment of borehole shape calculation model of horizontal well provides a basis for calculating the friction between the pipe string and the horizontal section.

Strategies for mud-removal from washouts during cementing of vertical surface casing

In the context of both production and well abandonment, effective cementing of the surface casing is key in preventing leakage of formation fluids. Although there are general guidelines for the effective cementing of vertical casings, there is little known about the effects of wellbore irregularity, e.g. from formations that are badly eroded during drilling - washouts. Here we present a systematic study of the cementing of a vertical surface casing with a washout section. We study the effects of washout length, pump rate, positive density difference and viscosity ratio, all in the displacement flow of the drilling mud by a spacer. Of these parameters, flow rate increases can lead to both improved displacements and to mixing and potential contamination. A positive density difference improves the mud displacement and viscosity differences have minimal effect, at least over the ranges studied. We then propose a pumping strategy that may improve the displacement process in a targeted washout, potentially valuable as we move into the era of real-time (and while drilling) well calipers and are progressively able to include irregular borehole geometries into cementing designs.

川西沙溪庙组不规则井眼高压气井固井技术

川西沙溪庙组不规则井眼高压气井固井技术

Hot-water coring system with positive displacement motor

In this study, a new coring system is designed that uses a positive displacement motor (PDM) connected to a core barrel and drill bit in combination with a hot-water drilling (HWD) system. The surface equipment and water hose are shared and the hot-water drill nozzle is replaced by the corer when a borehole is made by HWD. The hot water is then utilized to drive the PDM corer. In this paper, the concept of the PDM coring system is presented, along with a novel self-adaptable anti-torque device that can automatically adjust the diameter to adapt to an irregular borehole. The required force-energy parameters of the PDM are theoretically estimated. A PDM coring system is designed and assembled, and the feasibility of the concept is tested in the laboratory. The results indicate that it is more conducive to perform the coring at a lower temperature of water. At a temperature of 5 °C, a double-walled core-barrel works adequately to produce ice cores with a maximum diameter of 73 mm and an inner barrel diameter of 79 mm at a water flow rate of 96 L/min.

Understanding Loss Mechanisms: The Key to Successful Drilling in Depleted Reservoirs?

Summary Mud losses are frequently observed when drilling in depleted formations. This is because of the decrease in the minimum in-situ stress during depletion. As a result of this decrease, the lost-circulation pressure—or fracture gradient (FG)—decreases, and the operational mud-weight window shrinks. Losses in such formations are often observed when drilling through sand/shale sequences. Preventing and curing losses requires a sound understanding of loss mechanisms. In this study, we investigate several mechanisms that might be responsible for the elevated risk of mud losses in differentially depleted sand/shale sequences. Numerical models of synthetic cases representative of lost-circulation scenarios in a high-pressure/high-temperature (HP/HT) field in the North Sea, under normal faulting conditions, are set up using the finite-element method. The simulations reveal that shear displacement at the horizontal sand/shale interfaces is unlikely to cause losses. On the other hand, shear displacement and losses might be induced at high-angle sand/shale interfaces, such as those found near faults. In addition, faults introduce an extra complexity to the stress distribution and stress-path coefficients. Stress anisotropy near faults might increase during depletion, making both lost-circulation issues and borehole-stability problems worse. The zone most prone to lost circulation in a faulted formation is located in depleted sand adjacent to the fault. The loss mechanism here is because of drilling-induced fractures (DIFs). In addition, depletion itself might induce fractures in shale adjacent to the depleted sand and located across the fault from it. Such fractures might then serve as escape paths for the drilling fluid during infill drilling. The loss mechanism here is caused by pre-existing, depletion-induced fractures. These findings are in agreement with field observations. A noncircular (elliptic or irregular) borehole cross section is found to reduce the fracture-initiation pressure (FIP). An irregular borehole cross section is, thus, another possible mechanism behind irregular loss patterns observed in depleted fields. The results from the study are important for establishing best practices when drilling in depleted formations.

Application of Dipole Array Acoustic Logging in the Evaluation of Shale Gas Reservoirs

In order to effectively evaluate shale gas reservoirs with low porosity, extra-low permeability, and no natural productivity, dipole array acoustic logging, which can provide various types of information including P-wave slowness (DTC) and S-wave slowness (DTS), is widely used. As the dipole array acoustic logging tool has a larger investigation depth and is suitable for complex borehole environments, such as those with a high wellbore temperature, high drilling fluid column pressure, or irregular borehole wall, it has been mainly applied to the evaluation of lithology, gas potential, fractures, and stimulation potential in shale gas reservoirs. The findings from a case study of the Sichuan Basin in China reveal that the acoustic slowness, S-P wave slowness ratio (RMSC), and S-wave anisotropy of the dipole array acoustic logging can be used to qualitatively identify reservoir lithology, gas potential, and fractures. Using the relationship between DTC and the total porosity of shale gas reservoirs, and combined with the compensated neutron (CNL) and shale content (Vsh) of the reservoir, a mathematical model for accurately calculating the total porosity of the shale gas reservoir can be established. By using the relationship between the RMSC and gas saturation in shale gas reservoirs and tied with density log (DEN), a mathematical model of gas saturation can be established, and the determination of gas saturation by the non-resistivity method can be achieved, delivering a solution to the issue that the electric model is not applicable under low resistivity conditions. The DTS, DTC, and DEN of shale can be used to calculate rock mechanic parameters such as the Poisson’s ratio (POIS) and Young’s modulus (YMOD), which can be used to evaluate the shale stimulation potential.

Numerical Simulation for the Dynamic Breakout of a Borehole Using Boundary Element Method

Borehole breakouts are formed by spalling of fragments in a direction parallel to the least horizontal stress. As the breakouts falling into the wellbore or being scoured by the drilling mud piece by piece, the stress concentration on the wellbore will release gradually. The wellbore is not circular or elliptical, which requires a numerical method to determine the redistributed stress around the irregular borehole. In this article, boundary element method associated with the brittle rock failure criterion is deployed to simulate the dynamic breakout of a borehole. The borehole shape is updated step by step during the time-dependent breakout. The first breakout volume is smaller than the analytical solution (Gholami et al. in J Rock Mech Geotech Eng 8:521–532, 2016). However, the total breakout volume is larger than the analytical solution because our model considers the subsequent failure of the irregular wellbore. The stress distribution around the irregular wellbore after each step of breakout is investigated and the maximum cohesive strength at which the rock will fail is figured out. These results provide new insights to understand the dynamic process of borehole breakout.

Mechanical Analysis of Drill String Drag and Torque With the Condition of Irregular Borehole

With the rapid development of drilling technology, air drilling technology and large displacement well, and horizontal well drilling technology are also known as the main directions of the future drilling development. Compared with conventional drilling mode, air drilling is particular, such as drilling fluid with low density, poor lubrication, large friction coefficient between the drill string and borehole, irregular borehole, well trajectory control difficulty, and so on. Along with the change of drilling condition, the stress of drill string is also changed. However, irregular borehole has a direct impact on the fatigue failure and the stress of drill string. Therefore, it is necessary to analyze drill string stress and influence factors in irregular hole. Based on the previous studies, application of drill string mechanics and statistical regression methods is used to study drill string drag and torque, and it can predict the drilling string stress state with the hole enlargement rate, meanwhile, it also provides a theoretical basis for gas drilling and complex drilling technology.

Application of aerial photographs for the assessment of anthropogenic denudation impact on soil cover of the Brodnica Landscape Park plateau areas

The aim of the study was to assess suitability of colour aerial photographs for mapping of soil cover transformed due to effect of anthropogenic denudation. The investigation was carried out in south-western part of the Brodnica Landscape Park, within the boundaries of rolling and hilly moraine plateau, used for agricultural purposes. The soil cover of that area is exposed to intensive influence of slope processes triggered by human agricultural activity. The anthropogenic denudation leads to truncation of soil profiles of top convex sections of slopes and hills’ summits. Soil material moved down the slopes is accumulated in the form of diluvium in hollows and lower sections of slopes. Two study sites were selected – Sumówko and Zbiczno. Within the boundaries of both study sites, detailed soil mapping took place consisting in preparation of irregular boreholes projection. Next, four sites were selected for soil pits, representing broad spectrum of transformations related to anthropogenic denudation. Based on obtained results and colour diversity of surface horizons, the spatial range of individual soil types was specified. It also enabled determination of anthropogenic denudation impact on formation of the soil cover. Totally eroded soils, classified as pelosols, located on hills’ summits, are characterized by very bright colours of surface horizons, resulting from content of calcium carbonate in glacial tills. The range of soil lessivés, prevailing within the slopes boundaries, where the erosion resulted in exposure of argic horizons rich in iron compounds and clay fraction, coincided with occurrence of brown colours. Bright grey surface horizons are characteristic of deluvial soils. This colour arises from sandy texture of deluvial material (low content of iron) in combination with humus nature. The soils located in relatively vast field depressions were covered with small thickness of diluvium, which was reflected in dark grey colours of surface horizons. These horizons are relatively rich in soil humus. Significant amounts of humus are related with mix of deluvial material with material formed in humus horizons, originally occurring on surfaces of soils rich in organic matter – black earths and organic soils.

Removal of Cyclic Borehole Noise From Low- and High-Resolution LWD Images and Its Impact on Image Interpretation

Summary The use of logging-while-drilling (LWD) imaging tools in realtime decision making and post-drilling analysis has become commonplace. However, image noise and processing errors because of inherent measurement physics can propagate errors and, thus, complicate interpretation of openhole log data and images. For example, standard density images tend to amplify image noise from small borehole irregularities. Among different borehole irregularities, spiraling is known to occur more frequently with conventional rotary assemblies, steerable motor assemblies, and rotary-steerable assemblies. When the cyclic-noise amplitude from spiraling becomes large relative to the measurement of the primary interest, it grossly affects the quality of the recorded formation bulk density, photoelectric factor, and neutron porosity. This paper shows novel methods to remove cyclic noise from formation-evaluation (FE) images by applying frequency-domain filtering. Although the initial attempt of fast Fourier transforms (Zhang et al. 2010) illustrates the straightforward concept, it is seldom used because of implementation issues requiring interactive filter design and intensive operator intervention. Recently, a new method (Sugiura et al. 2011) has been designed to improve the filtering process. Additionally, the new adaptive filter designs automate the cyclic-noise-removal process from the FE images. Standalone software has been developed to process the entire image logs from one well, without human supervision. The software adaptively modifies the filter behavior as borehole-oscillation noise characteristics change with formation, drilling assemblies, hole inclination, and depth. To examine its validity qualitatively, the algorithm is then applied to various field data, not only on low-resolution density images but also on higher-resolution density images. The new algorithm has been proved by bringing considerable improvement to the image quality without any artificial interruptions. The rugosity effect is reduced significantly, and the apparent resolution of bed-boundary features is increased. Furthermore, comparative field examples illustrate the improvement in image-feature extraction. The method described here is critical, for example, for small-fracture detection and more-precise definition of formation-property contrasts indicative of bed boundaries in irregular boreholes. This new method not only is effectively used for density images but also is applicable to any other borehole images, such as resistivity and ultrasonic images.

3-D wave propagation in fluid-filled irregular boreholes in elastic formations

Different seismic testing techniques rely on the propagation of acoustic waves in fluid-filled boreholes from sources placed within the borehole and in the solid media. The interpretation of the signals recorded relies on understanding how waves propagate in the borehole and its immediate vicinity. It is known that very complex wave patterns can arise, depending on the distance between the source and the receiver, and their placement and orientation relative to the axis of a circular borehole. The problem becomes more complex if the cross-section is not circular, conditions for which analytical solutions are not known. In this work, the Boundary Element Method (BEM) is used to evaluate the three-dimensional wave field elicited by monopole sources in the vicinity of a fluid-filled borehole. This model is used to assess the effects of the receiver position on the propagation of both axisymmetric and non-axisymmetric wave modes when different borehole cross-sections are used. Both frequency vs. axial-wave number responses and time-domain responses are calculated.

Modeling of low‐frequency Stoneley‐wave propagation in an irregular borehole

A fast modeling method is formulated for low‐frequency Stoneley‐wave propagation in an irregular borehole. This fast modeling method provides synthetic waveforms which include the effects of two borehole irregularities, diameter changes (washout), and formation property changes. The essential physics of the low‐frequency Stoneley waves are captured with a simple 1-D model. A mass‐balance boundary condition and a propagator matrix are used to express Stoneley‐wave interactions with the borehole irregularities. The accuracy of the proposed method was confirmed through comparison with existing finite‐difference and boundary integral modeling methods that yielded cross‐correlations greater than 0.98. Comparison of synthetic records calculated for an actual borehole with field records showed qualitative agreement in the major reflections because of the washout zones, but showed some disagreements in the reflections caused by the fractures. Since the synthetic records include only information relating to the borehole geometry and the elastic properties of formation, the reflection caused by the fracture will appear only in the field record. These results suggest the possibility of distinguishing Stoneley‐wave reflections caused by fractures from those caused by borehole irregularities. Further, the fast computational speed of this method—over 300 times faster than either boundary integral or finite‐difference methods—makes it quite suitable for field application.

Multipole borehole acoustic waveforms: Synthetic logs with beds and borehole washouts

We give a two‐dimensional (2-D) velocity‐stress finite‐difference (vs-fd) formulation for the multipole borehole acoustic logging problem. Irregular boreholes, horizontal bedding, and axially varying alteration are encompassed by the model. Excellent agreement is obtained with transform techniques for constant radius boreholes and homogeneous formations. The method is used to generate waveforms for many positions of transmitter and receiver array relative to beds and washouts as in logging. First motion and semblance techniques are applied to the waveforms to extract formation slownesses, resulting in synthetic multipole logs which demonstrate the response of ideal multipole logging tools and signal processing to several environmental effects. For monopole, compressional, first‐motion logs, residual slowness errors remain at borehole washouts after borehole compensation. These errors increase as the effective measure point in the waveforms is moved back in time from true first motion, admitting greater interference from reflected and mode‐converted waves. Errors at bed boundaries are typically smaller than those at washouts. Stoneley slowness logs are obtained by narrow‐band filtering of low frequency monopole waveforms and application of the semblance slowness extraction algorithm STC. Similar processing applied to low frequency dipole waveforms yields flexural mode slowness logs. A small correction satisfactorily accounts for flexural mode dispersion properties, yielding formation shear slowness. Slowness errors for both Stoneley and dipole shear logs at washouts and bed boundaries are quite small, typically on the order of a few percent. Borehole compensation is of marginal benefit for both of these logs since they are based on modes of the borehole rather than head‐waves.

To: “Full wave logging in an irregular borehole” by Michel Bouchon and Denis P. Schmitt (GEOPHYSICS, 54, 758–765, June 1989)

The boundary‐integral equation formulation described by Bouchon and Schmitt is exact, but the derivation of the formulas for the components of the displacement vector in the formation due to circular radial force are erroneous. The correct derivation follows.