Borehole Stability Research Articles

Borehole Instability in Malmberget Underground Mine

Borehole instability causes borehole failure, presenting a challenge to the drilling industry (Muller et al. 2009; Zhang 2013). The borehole stability of Marcellus shale wells in long wall mining areas in southeast Pennsylvania, West Virginia, and eastern Ohio has been evaluated by Wang et al. (2014). Here, the ground deformation caused by coal mining generates large ground movement and creates complex stress changes in subsurface rock. This, in turn, triggers interruptions in the operation of the borehole well causing safety and environmental concerns. Borehole walls may fail when the surrounding stress exceeds the tensile, the compressive, or the shear strengths of the rock formation, whichever is reached first (Zhang et al. 2003). Horizontal well borehole stability is analyzed by the new in situ stress prediction model in shale gas reservoirs (Yuan et al. 2013). Besides this, in hard rock underground mining, such as sublevel caving (SLC), uncharged and undetonated blast holes reduce specific charge, result in poor fragmentation and may even lower ore recovery (Zhang 2005). It is often found that boreholes are blocked by stones and the charge work has to stop in the field. In many SLC rings two or three boreholes in each ring are often broken or blocked by either stones or pieces of concrete according to the mine’s production archives that contain daily notes on various production problems met by the miners. In order to get detailed information on the borehole problems occurring in LKAB’s two mines, a preliminary investigation was carried out by Kangas (2007) with a mini-video camera, showing that typical borehole problems were borehole deformation and boreholes jammed by stones. Due to various reasons, the above investigation rested for a while. This study is a follow-up of the previous investigationmade by Kangas (2007) in more ore bodies and more boreholes. The main objective of this study is to identify various instability problems of boreholes by field filming in different ore bodies in the mine.

Borehole Stability Analysis in Deepwater Shallow Sediments

Deepwater shallow sediment is less-consolidated, with a rock mechanical behavior similar to saturated soil. It is prone to borehole shrinkage and downhole leakage. Assume the deepwater shallow sediments are homogeneous, isotropic, and ideally elastoplastic materials, and formation around the borehole is divided into elastic and plastic zone. The theories of small deformation and large deformation are, respectively, adopted in the elastic and plastic zone. In the plastic zone, Mohr–Coulomb strength criterion is selected. The stress and deformation distributions in these two zones, and the radius of plastic zone are derived. The collapse pressure calculation formula of deepwater shallow sediments under the control of different shrinkage rates is obtained. With the introduction of excess pore pressure theory in soil mechanics, the distribution rule of excess pore pressure in these two zones is obtained. Combined with hydraulic fracturing theory, the fracture mechanism of shallow sediments is analyzed and the theoretical formula of fracture pressure is given. The calculation results are quite close to the practically measured results. So the reliability of the theory is confirmed.

Development and applications of rock fracture mechanics modelling with FRACOD: a general review

Rock failure is often controlled by fracture initiation, propagation and coalescence, especially in hard rocks where explicit fracturing rather than plasticity is the dominant mechanism of failure. Prediction of the explicit fracturing process is therefore necessary when the rock mass stability is investigated for engineering purposes. However, the fracture mechanics approach is rarely used in practical rock engineering design partly due to the inadequate understanding of complex fracturing processes in jointed rock mass and partly due to the lack of tools which can realistically predict the complex fracturing phenomenon in rock mass. Since 1990s, a new approach to simulating rock mass failure problems has been developed using a numerical code namely FRACOD. FRACOD is a code that predicts the explicit fracturing process in rocks using fracture mechanics principles. Over the past three decades, significant progress has been made in developing this approach to a level that it can predict actual rock mass stability at an engineering scale. The code includes complex coupling processes between the rock mechanical response and thermal process and hydraulic flow, making it possible to handle coupled problems often encountered in geothermal, hydraulic fracturing, nuclear waste disposal and underground Liquefied Natural Gas (LNG) storage. During this period, numerous application cases have been conducted using FRACOD, which includes: borehole stability in deep geothermal reservoir, pillar spalling under mechanical and thermal loading; prediction of tunnel and shaft stability and excavation disturbed zone, etc. This paper summarises the theoretical fundamentals of the fracture mechanics approach with FRACOD and the most recent developments. Several selected application cases are also discussed briefly in this paper to demonstrate the effectiveness of this approach.

Study on the Borehole Stability in Liulin Coal Reservoirs

This paper studied the borehole stability in Liulin bituminous core reservoirs during under-balanced drilling. The equivalent depth method was applied to study the formation pore pressure. The floor drain test method was applied to study the ground stress. Core strength experiments combined with logging data were used in the establishment of core strength predicting model. Mechanics analysis model was founded to study the borehole stability in the core reservoir. According to the studying and analyzing results, the under-balance drilling in Liulin coal reservoirs enjoys good borehole stability.

Study of the Effect of Borehole Size on Wellbore Stability

Based on the size effect of rock strength, the borehole stability analysis model is established, which the borehole size is taken into consideration. Through this model, the relation between borehole size and collapse pressure under borehole pressure, ground stress and drilling fluid flow function is analyzed. The analysis shows that with the increase of borehole diameter, collapse pressure increases significantly, and borehole stability becomes poor, but the variation of borehole size is not proportional to collapse pressure: the bigger the borehole, the smaller the variation. When borehole diameter increases from 152.4 mm to 444.5 mm, wellbore collapse pressure increases from 1.18g/cm3 to 1.315g/cm3 and the rate of the increase is 11.44%. When slimhole drilling technology is applied, the density of minimum fluid that maintains wellbore stability is lower than the one used in conventional wellbore drilling.

Borehole Stability in Shale Formation: Modeling the Effects of Molecular Weight and Concentration of Polymers in the Drilling Fluids Formulation

A new experimental model was developed to predict the role of special polymeric additives, in the drilling fluid formulation, on the wellbore stability in shale formation. The shale formation was regarded as a non-ideal membrane and the effects of various characteristics of the added polymers were studied on the membrane reflection coefficient. The model was applied to unique field data from the oil field in south of Iran, including clay structure, cation exchange capacity (CEC), density and porosity of the shale. The results, using various polyglycols and polyacrylamides as the polymeric additive, showed that the structure of the polymeric chains e.g. type and content of ionic segments had significant effect on their adsorption mechanism and its strength. It was concluded that increasing the molecular weight of the polymer chains decreased the rate and amount of the adsorption due to the increasing of the entanglements between the chains which in turn limited their mobility. So, adsorption of the polymeric material on the shale had significant impress on its performance as a membrane by increasing the shale reflection coefficient enhancing its stability during drilling process. Finally, the developed model results were in good agreement by experimental test results which was done in a specific shale stability set up.

Finite Element Simulation of Freezing Technology for Borehole Stability of Gas Drilling

The gas drilling mainly relies on the high speed air flow to carry the cuttings. The formation water or oil mixed with the cuttings and then they stick together in clumps after the formation water or oil went into the hole annulus, the clumps stick on the drill string and the borehole. The clumps may block the hole annulus and cause the stick or bury the drill string and many other complex accident. It could stop the cuttings from sticking with the liquid through freezing the formation fluid with the liquid nitrogen. And the natural geotechnical becomes into the frozen soil, and forms the temporary solid which is intact, high strength and low-permeability. This paper utilize the ANSYS finite element program to simulate the 3D model of borehole and hole wall to calculate the freezing radius of the steady state, heat loss, temperature of the freezing point and the conductive heat time of the unsteady state. And this study has provided the basis of the freezing technology for borehole stability of gas drilling.

Freezing Technology for Borehole Stability of Gas Drilling

The gas drilling mainly relies on the high speed air flow to carry the cuttings. The formation water or oil mixed with the cuttings and then they stick together in clumps after the formation water or oil went into the hole annulus, the clumps stick on the drill string and the borehole. The clumps may block the hole annulus and cause the stick or bury the drill string and many other complex accident. It could stop the cuttings from sticking with the liquid through freezing the formation fluid with the liquid nitrogen. And the natural geotechnical becomes into the frozen soil, and forms the temporary solid which is intact, high strength and low-permeability. This technology could achieve the purpose of strengthening the formation and reducing the fluid flow of the formation, and it greatly broadens the scope of application of gas drilling.

Scheme Optimization of Freezing Technology for Borehole Stability of Gas Drilling

The gas drilling mainly relies on the high speed air flow to carry the cuttings. The formation water or oil mixed with the cuttings and then they stick together in clumps after the formation water or oil went into the hole annulus, the clumps stick on the drill string and the borehole. The clumps may block the hole annulus and cause the stick or bury the drill string and many other complex accident. It could stop the cuttings from sticking with the liquid through freezing the formation fluid with the liquid nitrogen. And the natural geotechnical becomes into the frozen soil, and forms the temporary solid which is intact, high strength and low-permeability. In this paper, according to the characteristic of the gas drilling technology, we optimized the scheme of freezing technology for borehole stability to give the theoretical basis of the industrial application.

Directional drilling in unstable environments

Directional drilling has been established in the coal industry as a viable means of gas drainage, exploration and water management. But the environment in and around coal seams is not always conducive to stable conditions while drilling and borehole stability after the drilling has been completed. This paper identifies the conditions which cause unstable drilling conditions and the various means which are used to attempt to manage or bypass those conditions. Ultimately, equipment does become bogged in these adverse environments and requires recovery by over-coring.

The Role of Shear Failure on Stress Characterization

Leak-off pressure and lost circulation data are generally thought to be reflective of minimum stress. We propose an alternative interpretation should be considered where the data may reflect a shear failure along zones of pre-existing weakness rather than opening of tensile fractures against the minimum stress. This mechanism has been discussed in a small number of borehole stability and hydraulic fracture papers, but has not been widely applied to leak-off test or lost circulation interpretation. In this paper, we will revisit and expand the concept introduced recently by Couzens-Schultz and Chan (J Struct Geol, doi: 10.1016/j.jsg.2010.06.013, 2010) based on abnormally low leak-off tests in an active thrust belt to the analysis of lost circulation observations in modern-day deltaic environments. In the Gulf of Mexico, lost circulations historically are interpreted as a representation of the minimum horizontal stress due to initiating or reopening of a fracture in tensile mode. However, shear failure or fault reactivation can occur at pressures well below the minimum far-field stress that is typically considered a safe upper bound for mud pressure if pre-existing planes of weakness such as faults or fracture networks exist. We demonstrated a mud loss event is shown to be inconsistent with the tensile failure mode in a normal stress environment, but in good agreement with expectations for shear failure along pre-existing faults.

Numerical and experimental study of the stability of non-circular boreholes in high-permeability formations

One innovative drilling concept explored in recent years is based on the use of a non-circular well cross section, in particular a well with grooves running along the well axis. The grooves are expected to improve wellbore hydraulics and hole cleaning. In the present study, borehole stability analysis has been carried out for a horizontal rifle well in a high-permeability formation. It has been found that tensile failure is likely to develop at several locations around the non-circular cross section, a failure mode that is not normally found in conventional (circular) wells. The numerical results obtained with a finite-element code have been qualitatively confirmed in a dedicated laboratory experiment performed on a block of concrete with a section of non-circular well. In the experiment, the unusual failure mode was clearly observed. Traditional breakouts were also observed in the same test, but had smaller size. Numerical simulations have been carried out for different drilling conditions, both onshore and offshore, and depths from 1000m to 2000m, normal or abnormal pore pressure. The borehole was always found to fail in tension unless a considerable overbalance was applied by the drilling fluid. Similar results were obtained with formations that had hydrostatic pore pressure, elevated pore pressure (overpressure) or reduced pore pressure (underpressure, depleted reservoirs). The results are of importance for practical application of non-circular wells.

Borehole Stability in Shale Formation for Extended Reach Wells

In recent years, Extended reach well (ERW) drilling technology is widely used in the offshore oil & gas fields in order to reduce the number of the drilling platforms. As it has notable characteristics such as the high deviation angle, large horizontal displacement and long open borehole interval the borehole stability increases the drilling risk and cost dramatically. To research the ERW borehole stability, including mechanical model, shale hydration test and the effect of circulating pressure loss in this paper, rock mechanics theory and hydraulics principle were comprehensively applied . The results show that, the safer drilling azimuth of the ERW in normal fault lies in the minimum horizontal principle stress direction; hydration radius increases with the passage of time, and the hydration collapsed rock has important influence on cutting beds and circulating pressure loss in annulus; the upper limit value of safety mud density decreases with the well depth increases, and when it deceases to the equivalent mud density of collapse pressure, the limit depth of the ERW is obtained. The research results provide relevant guidance on the ERW drilling, which can be used to determine the upper and lower safety mud weight limits, the best well trajectory selections and the well structure design.

Enhanced RSS Technology Pushes Drilling Envelope

Technology Update Increasing technical demands are pushing the envelope of rotary steerable system (RSS) drilling technology. The growing challenges of directional drilling in hard formations, borehole stability issues, difficult hole trajectories, and harsh downhole environments all are placing increasing demands on drilling and adding costs and risks to operations. RSS advancements have played a key role in pushing drilling into new frontiers such as deepwater and unconventional basins. As wells have become more complex and difficult, the industry seeks continuing advancements to enable the economic development of previously inaccessible resources in a variety of environments. The Schlumberger PowerDrive Orbit enhanced RSS (Fig. 1) expands the ability to complete multiple functions in one run, tolerating higher revolutions per minute while maintaining good trajectory control, minimizing tortuosity, and accommodating challenging hydraulic designs and muds formulated for more aggressive drilling. Addressing Challenges The technology was engineered to address the most challenging drilling conditions and provide the reliability and efficiency needed to plan one run per section from shoe to total depth. The tool reduces the number of trips in and out of the hole, decreases the risk of differential and mechanical sticking, and improves hole cleaning and conditioning capabilities. The enhanced RSS system includes a newly designed pad actuator, thus increasing reliability and enhancing trajectory control by use of self-steering automation on any type of rig worldwide. The system also offers an optional, or on-demand, stronger pad design to provide greater durability in highly abrasive formations. In wells that typically would require two or three runs to drill one section, the new design enables operators to drill a section in a single run with improved performance. It also expands the revolution rate limits, tolerating up to 350 RPM, from the current limit of 220 RPM, while maintaining directional control and consistent steerability. Enhancing Existing Technology The new RSS system was designed to boost the effectiveness of existing technology. With an improved pad force, push-the-bit system for consistent dogleg severity at higher revolutions per minute, the new technology pushes the operating envelope of PowerDrive pushthe- bit systems. When combined with the PowerDrive vorteX powered RSS, it offers a wider tolerance for higher revolutions per minute and torque downhole.

Thermophysical and mechanical properties of granite and its effects on borehole stability in high temperature and three-dimensional stress.

When exploiting the deep resources, the surrounding rock readily undergoes the hole shrinkage, borehole collapse, and loss of circulation under high temperature and high pressure. A series of experiments were conducted to discuss the compressional wave velocity, triaxial strength, and permeability of granite cored from 3500 meters borehole under high temperature and three-dimensional stress. In light of the coupling of temperature, fluid, and stress, we get the thermo-fluid-solid model and governing equation. ANSYS-APDL was also used to stimulate the temperature influence on elastic modulus, Poisson ratio, uniaxial compressive strength, and permeability. In light of the results, we establish a temperature-fluid-stress model to illustrate the granite's stability. The compressional wave velocity and elastic modulus, decrease as the temperature rises, while poisson ratio and permeability of granite increase. The threshold pressure and temperature are 15 MPa and 200°C, respectively. The temperature affects the fracture pressure more than the collapse pressure, but both parameters rise with the increase of temperature. The coupling of thermo-fluid-solid, greatly impacting the borehole stability, proves to be a good method to analyze similar problems of other formations.

Effect of Gas Hydrate Drilling Fluids Using Low Solid Phase Mud System in Plateau Permafrost

Designingand optimizing suitable drilling fluids for permafrost exploration to get gas hydrates at low temperature is one of most essential and urgent demands in the current research on exploration and exploitation of gas hydrates. The issue on rheological properties and maintaining borehole stability of drilling fluids is especially important to focus on. This work studies the gas hydrates and wellbore stability conditions and designs a low solid phase mud system with chemical additives. The measured values and effect on rheological and physical properties of the drilling fluids with different type and concentration of additives are studied. Hydration expansion is tested in laboratory to estimate the hydration property of different additives. It is showed that the drilling fluids with a dose of 1% HT have suitable rheological and physical properties at low temperature. Potassium ion performs well in preventing borehole instability and gas hydrate dissociation. According to the study, the optimized drilling fluid formula is: base mud + 10% NaCl + 5% KCl + 1 ‰ NaOH + 1% HT.

Development of High Efficient Anti-sloughing Drilling Fluid and Its Application in Chunhua Block

Chunhua Block of Shengli Oilfield is one block where shale caving formation, borehole instability, well click, trip sticking, drill pipe blocking and sticking during completion logging and other downhole complex issues were encountered during the drilling process. The paper was based on slow down pressure transmission, filtrate invasion and strong inhibition of cooperation anti-sloughing basic principle. Through optimization and compatibility of the major additives, a new-type drilling fluid is characterized by good rheological properties, filtration loss, lubricity, stability and strong inhibition. The new drilling fluid has been applied in 5 wells for block test. The result showed that all wells gained a satisfactory anti-sloughing effect. The enlargement ratio of well bore diameter was reduced efficiently. The drilling time was saved. And the ratio of successful logging by one time was 100%. Key words: Drilling fluid; Borehole stability; Shale strata; Cooperation anti-sloughing; Chunhua Block

Research on borehole stability of shale based on seepage-stress-damage coupling model

In oil drilling, one of the most complicated problems is borehole stability of shale. Based on the theory of continuum damage mechanics, a modified Mohr-Coulomb failure criterion according to plastic damage evolution and the seepage-stress coupling is established. Meanwhile, the damage evolution equation which is based on equivalent plastic strain and the permeability evolution equation of shale are proposed in this paper. The physical model of borehole rock for a well in China western oilfield is set up to analyze the distribution of damage, permeability, stress, plastic strain and displacement. In the calculation process, the influence of rock damage to elastic modulus, cohesion and permeability is involved by writing a subroutine for ABAQUS. The results show that the rock damage evolution has a significant effect to the plastic strain and stress in plastic zone. Different drilling fluid density will produce different damage in its value, range and type. This study improves the theory of mechanical mechanism of borehole collapse and fracture, and provides a reference for the further research of seepage-stress-chemical-damage coupling of wall rock.

Estimation of Fracture Stiffness, In Situ Stresses, and Elastic Parameters of Naturally Fractured Geothermal Reservoirs

AbstractKnowledge of fracture stiffness, in situ stresses, and elastic parameters is essential to the development of efficient well patterns and enhanced geothermal systems. In this paper, an artificial neural network (ANN)–genetic algorithm (GA)-based displacement back analysis is presented for estimation of these parameters. Firstly, the ANN model is developed to map the nonlinear relationship between the fracture stiffness, in situ stresses, elastic parameters, and borehole displacements. A two-dimensional discrete element model is used to conduct borehole stability analysis and provide training samples for the ANN model. The GA is used to estimate the fracture stiffness (kn, Ks), horizontal in situ stresses (σH, σh), and elastic parameters (E, v) based on the objective function that is established by combining the ANN model with monitoring displacements. Preliminary results of a numerical experiment show that the ANN-GA-based displacement back analysis method can effectively estimate the fracture stif...

Investigation of borehole stability in poorly cemented granular formations by discrete element method

Abstract Behaviour of poorly cemented formations in case of drilling a vertical exploration borehole will be studied to achieve an in-depth understanding of borehole stability problem. Analysis of the granular formation behaviour has a significant importance in identifying stability issues, designing adequate borehole supports and choosing an efficient drilling method. This paper presents numerical investigations on the behaviour of poorly cemented formations in the vicinity of an unsupported vertical cylindrical borehole. Due to poor cementation and therefore granular behaviour of these formations, Discrete Element Method (DEM) was identified as being well suited for developing realistic models. To conduct the numerical studies a cube of 8 m 3 made up of spherical particles with diameters ranging from 5 mm to 70 mm was constructed and analysed in three-dimensional Particle Flow Code ( PFC 3 D ). It is a discontinuum code used in analysis of the granular materials where the interaction of discrete grains is considered. A cylindrical opening with the diameter of 0.3 m runs along the central vertical axis of the cube simulating the presence of a borehole. The stresses applied to the cube simulate the underground conditions around an exploration borehole at the depth of 80 m. The effects of in situ stresses around the borehole, strength of particle bonding and fluid flow pressure on the stability of the formation around the borehole have been investigated. It has been shown that the development of in situ stresses in the ground due to drilling a borehole results in the formation of a plastic zone around that borehole. When there is lack of sufficient bonding between the sand grains, the interaction between them results in their movement towards the borehole opening and thus eventuates the collapse of the borehole wall. Furthermore, the presence of high pressure water flow expedites the process of the borehole collapse.