Bottom-hole Rock Research Articles

Poroelastic modeling of cutting bottom-hole rock – Part I: Stress state of bottom-hole rock

This is the first part of a series of two papers modeling cutting bottom-hole rock during drilling. This paper studies the stress state of a bottom-hole rock that a downhole bit is drilling. A bottom-hole rock underground is subjected to far field in-situ stresses. During drilling process, the rock materials above the bottom-hole rock are gradually removed and replaced with drilling fluid, thus altering the stress state of the bottom-hole rock.In this paper, a model is developed based on the theory of Linear Poroelasticity to calculate the evolution of stresses and pore pressure in a bottom-hole rock during drilling process. The bottom-hole rock under consideration can be selected at any depth underground. To solve the model, superposition principle and finite difference method are employed. The model is also extended for inclined wellbores in arbitrary directions.A series of case studies with various drilling parameters in different formations are presented. The results show that during drilling bottom-hole rock will expand, leading to the pore pressure decrease if formation has a low permeability. The pore pressure decrease in a bottom-hole rock will increase differential pressure, eventually increasing difficulties in drilling. The results show that low permeability formation drilled with a greater rate of penetration will lead to a lower pore pressure in a bottom-hole rock. Moreover, during air drilling in hard formation, the pore pressure decreases significantly in bottom-hole rock.

Numerical modelling of bottom-hole rock in underbalanced drilling using thermo-poroelastoplasticity model

Stress analysis of bottom-hole rock has to be considered with much care to further understand rock fragmentation mechanism and high penetration rate. This original study establishes a fully coupled simulation model and explores the effects of overburden pressure, horizontal in-situ stresses, drilling mud pressure, pore pressure and temperature on the stress distribution in bottom-hole rock. The research finds that in air drilling, as the well depth increases, the more easily the bottom-hole rock is to be broken. Moreover, the mud pressure has a great effect on the bottom-hole rock. The bigger the mud pressure is, the more difficult to break the bottom-hole rock is. Furthermore, the maximum principal stress of the bottom-hole increases as the mud pressure, well depth and temperature difference increase. The bottom-hole rock can be divided into three main regions according to the stress state, namely a) three directions tensile area, b) two directions compression areas and c) three directions compression area, which are classified as a) easy, b) normal and c) hard, respectively, for the corresponding fragmentation degree of difficulty. The main contribution of this paper is that it presents for the first time a thorough study of the effect of related factors, including stress distribution and temperature, on the bottom-hole rock fracture rather than the well wall, using a thermo-poroelastoplasticity model.

Rock failure by thermal stress in gas drilling

The factors that contributing high penetration rate of gas drilling is complex. Isentropic flow was generated when gas pass through bit nozzle during gas drilling. This phenomenon would lead to cryogenic effects. The thermal stress on the bottom hole rock was formed by the effect, which can reduces the strength of the rock and promotes the mechanical rock breaking. First, the temperature distribution model of bottom hole rock under the condition of asymmetric cooling was built in the paper. The three-dimensional dynamic thermal shock stress distribution model was established based on the temperature model. Then, the change of the rock cohesion was analysed by using the Mohr Coulomb criterion. The results demonstrate that with the temperature decreases, the strength of rock is greatly reduced, can improve ROP. Last, the liquid nitrogen cooling test and real time measurement of acoustic wave were conducted to verify the above analyse. The First wave amplitude has a dramatic delay, which illustrate the cooling have an important impact on rock internal structure. The paper clearly depicts the mechanism of rock failure under the condition of dynamic low temperature in gas drilling. [Received: September 7, 2017; Accepted: June 20, 2018]

Numerical simulation on downhole flow and temperature fields during drilling with nitrogen jet

In petroleum engineering, nitrogen drilling is an important technology for building wellbores between surfaces and reservoirs. To uncover the downhole flow field and the change rules of rock temperature during drilling with nitrogen jet, we constructed a CFD model by coupling the property equations of nitrogen. The flow fields of nitrogen jet and rock temperature distribution at different times were simulated. Results showed that the high speed nitrogen jet can be efficiently generated because of the nozzle acceleration and the impingement effect can be induced during drilling. The temperature of the nitrogen jet decreased due to the Joule-Thomson effect. This phenomenon suggested that the nitrogen jet induced additional thermal cracks on the bottomhole rock, which was very beneficial for the improvement of rock-breaking efficiency.

Thermal characteristics analysis with local thermal non-equilibrium model during liquid nitrogen jet fracturing for HDR reservoirs

This paper presents a numerical analysis of thermal characteristics in bottom-hole rock during liquid nitrogen jet fracturing for hot dry rock (HDR) reservoirs. The standard κ-ε and Darcy models are employed to calculate the fluid flow in fluid region and porous-rock region respectively. The heat transfer between flowing fluid and rock matrix is simulated by the local thermal non-equilibrium model. The thermal stresses are evaluated based on thermo-elastic model. Furthermore, a sensitivity study is conducted to investigate the influences of different factors on heat transfer efficiency in porous rock. The results show that the fluid has a relatively high velocity in the rock region impacted by high-speed jet. The flow transport in porous rock can significantly improve the cooling performance of hot rock. The thermal stresses induced by temperature gradient are huge enough to create thermal cracks in formation. The parametric analysis indicates that increasing the values of ambient pressure, rock permeability and convection coefficient can promote the heat transfer in porous rock obviously, while the parameters of pressure drop and rock porosity have a slight impact on the cooling rate. The results in this study would offer guidance for the design of HDR fracturing with liquid nitrogen jet.

Downhole Transient Flow Field and Heat Transfer Characteristics During Drilling With Liquid Nitrogen Jet

As a novel jet technology, liquid nitrogen jet (LNJ) is expected to effectively break rocks and further provide a high-efficiency method for drilling, especially geothermal drilling. Using this technology, rocks can be broken down by the coupled effects of cryogenic cooling and jet impingement. In this study, transient downhole jet flow field and heat transfer during drilling with LNJ were simulated. Then, the distributions of temperature (including LNJ and ambient rock), velocity, and pressure at different times were analyzed. Finally, the effects of the parameters on jet impingement and rock cooling performance were discussed. Results indicated that cryogenic LNJ could be efficiently generated in the downhole region. The temperature of the rock surface remarkably decreased as the LNJ reached the bottomhole. The high-speed LNJ caused axial impingement and radial shear effects on the bottomhole rock. The rock cooling performance caused by the LNJ was influenced by the initial rock temperature. With the increase of the initial rock temperature, the drop amplitude of the rock temperature also increased. The impingement capability of the LNJ was improved by increasing the nozzle diameter and the nozzle pressure drop. With the increase of standoff distance, the wall pressure and the radial velocity of the bottomhole decreased while increasing the impingement scope. The confining pressure hardly influenced the rock cooling performance and jet impingement capability, thereby indicating that LNJ could work even at high confining pressure conditions.

Stress distribution and its influencing factors of bottom-hole rock in underbalanced drilling

The underbalanced drilling has been widely used due to its advantages of high drilling efficiency and low cost etc., especially for hard formation drilling. These advantages, however, are closely related to the stress state of the bottom-hole rock; therefore, it is significant to research the stress distribution of bottom-hole rock for the correct understanding of the mechanism of rock fragmentation and high penetration rate. The stress condition of bottom-hole rock is very complicated while under the co-action of overburden pressure, horizontal in-situ stresses, drilling mud pressure, pore pressure and temperature etc. In this paper, the fully coupled simulation model is established and the effects of overburden pressure, horizontal in-situ stresses, drilling mud pressure, pore pressure and temperature on stress distribution of bottom-hole rock are studied. The research shows that: in air drilling, as the well depth increases, the more easily the bottom-hole rock is broken; the mud pressure has a great effect on the bottom hole rock. The bigger the mud pressure is, the more difficult to break the bottom-hole rock; the max principle stress of the bottom-hole increased with the increasing of mud pressure, well depth and temperature difference. The bottom-hole rock can be divided into 3 regions respectively according to the stress state, 3 direction stretch zone, 2 direction compression area and 3 direction compression zone; the corresponding fragmentation degree of difficulty is easily, normally and hardly.

The operational theory and experimental study of scraping-wheel diamond bit

The Scraping-wheel Diamond Bit is a new bit technology being put forward to solve the problems such as seriously uneven wear, thermal wear on the cutters of traditional diamond bits, especially PDC bits. The essential feature of this new technology is that the bit body is configured with rotational scraping-wheels with large angular deflection meanwhile the wheels are fixed with PDC compacts or other diamond cutters. When drilling in the formation, the scraping-wheel is driven to rotate slowly so that cutters thereon scrape the bottomhole rock successively rather than continuously. In this study, basic theory of geometry and kinematics of the Scraping-wheel Diamond Bit has been established, and variation law of wheel/bit speed ratio, tracks of cutters and main structure of the novel bit have been researched. Besides, an indoor simulated rock-breaking experiment for the Scraping-wheel Diamond Bit has been conducted, in which the influencing rule of the structure type and the structural parameters on cutters during the successive cutting process has been researched. Thus, significant experiment results, especially in the aspects of the variation law of wheel/bit speed ratio and the moving tracks of cutters, are achieved through this experiment, which not only proved the theory of geometry and kinematics of the novel bit, but also provided strong support for the rationality and feasibility of this new technology.

Kinematic and bottom-hole pattern analysis of a composite drill bit of cross-scraping

For improving the drilling efficiency of polycrystalline diamond compact drill bit, a novel polycrystalline diamond compact drill bit is presented with a new rock-breaking method named as cross-scraping. In the novel polycrystalline diamond compact drill bit which is referred to as a composite drill bit, polycrystalline diamond compact cutters are mounted on rotary wheels as major cutting elements, and as a result, mesh-like scraping tracks are formed in the outer radial area of the bottom-hole. Rock-breaking method of the composite drill bit causes both shearing and fracture failure of the bottom-hole rock, which will greatly increase the rock-breaking efficiency and will prolong the bit service life. By analyzing the complex motion of cutters on the composite drill bit, velocity and acceleration models of the cutters, as well as wheel/bit speed ratio model of the bit are established in accordance with the geometric relationship between cutters and bit body in a compound coordinate system. In simulation examples, motion tracks, velocity and acceleration features of the cutter and especially the bottom-hole pattern are analyzed. Further, indoor experiments are conducted to test the mesh-like bottom-hole pattern and rock-breaking features, which have proved the accuracy of the analysis model of the composite drill bit.

Study of the bottom-hole rock stress field under water jet impact

ABSTRACTOn the basis of analysis of the bottom-hole rock stress field under water jet impact, there are three types of coupling, which include the coupling of pore fluid and water jet, the coupling of pore fluid and rock matrix, and the coupling of water jet and rock matrix. The fluid-solid coupling model with the four main factors of three-dimensional in-situ stress, fluid column pressure, pore pressure, and water jet velocity is established and calculated by the finite element method and the finite volume method. The results show that the maximum principal stress of the bottom-hole increases with the increase of bottom-hole differential pressure. The maximum jet impact force is proportional to the square of the jet velocity; the pore pressures on the impact surface and along impact axis decrease in the form of cubic parabola with the increase of the distance. The local effect is obvious under water jet impact; the main affected area of the water jet is 2 times jet radius on the impact surface and 2.5 to 3.5 times jet radius along the impact axis when water jet velocity increases from 50 to 250 m/s, which is consistent with the stress wave attenuation theory. Study of the bottom-hole rock stress field under water jet impact provides a numerical research method for study of the water jet rock breaking mechanism under actual drilling conditions and the theoretical basis for faster and more efficient drilling.

Confined compressive strength model of rock for drilling optimization

Abstract The confined compressive strength (CCS) plays a vital role in drilling optimization. On the basis of Jizba's experimental results, a new CCS model considering the effects of the porosity and nonlinear characteristics with increasing confining pressure has been developed. Because the confining pressure plays a fundamental role in determining the CCS of bottom-hole rock and because the theory of Terzaghi's effective stress principle is founded upon soil mechanics, which is not suitable for calculating the confining pressure in rock mechanics, the double effective stress theory, which treats the porosity as a weighting factor of the formation pore pressure, is adopted in this study. The new CCS model combined with the mechanical specific energy equation is employed to optimize the drilling parameters in two practical wells located in Sichuan basin, China, and the calculated results show that they can be used to identify the inefficient drilling situations of underbalanced drilling (UBD) and overbalanced drilling (OBD).

A Study on the Effect of Bottom-hole Differential Pressure on the Rock Stress Field

Bottom-hole differential pressure is one of the most important factors that affects the drilling penetration rate. The purpose of the article is to study the effect of bottom-hole differential pressure on the vertical bottom-hole stress field. On the basis of mechanical analysis of the bottom-hole rock, the fluid-solid coupling model with the bottom-hole differential pressure is established under axisymmetry condition and is solved by using a numerical method. The comparison of the distribution of borehole wall stress between the numerical and theoretical solution is performed to verify the feasibility and rationality of the model. Then analysis of the stress state of the whole bottom-hole rock to break is carried out. The results show that the numerical solution is consistent with the theoretical solution and the fluid-solid coupling model is reasonable. With the increasing of the bottom-hole differential pressure, the bottom-hole tensile stress increases and compressive stress decreases and tensile region becomes larger. The whole bottom-hole rock to break is divided into three regions: triaxial tension region, biaxial compression and unidirectional tension region, and triaxial compression region. Qualitative and quantitative analysis of the bottom-hole stress field under different differential pressure provides the theoretical basis for rock breaking mechanism and faster and more efficient drilling.

Thermal stress on bottom hole rock of gas drilling

Gas drilling has higher penetration than mud drilling. The greatest reason for this phenomenon with gas is that the gas is greatly cooled by expansion as it passes through the bit and thereby cools the bottom of the hole. The thermal stress at bottom-hole occurs during this process. The concept of thermal crushing of rocks is analysed in this study. The theoretical methods are developed to analyse thermal stresses and fragmentation induced by cooling of rock. Then, the numerical computation is conducted for the thermal stress equations with the numerical result simulated for the temperature field at the bottom hole to explain the reason of high drilling rates in gas drilling. Furthermore, an experiment was conducted to verify the theory. Therefore, the theories and simulated results in this paper have a guiding signification for best understand the technique and possibly to extend its economic advantage still further.

Analytical and numerical modeling reveals the mechanism of rock failure in gas UBD

Underbalanced drilling (UBD) has been proven to be an effective means of increasing drilling performance and minimizing formation damage. While UBD with gases (air, nitrogen, natural gas, etc.) gives the highest rate of penetration (ROP), addition of liquid phase significantly reduces ROP. The objective of this study was to understand the mechanism of rock failure that controls the ROP and use the gained knowledge to optimize gas UBD design. An analytical model was developed in this study to calculate the near-wellbore rock stresses induced by the bottom hole pressure and temperature. A numerical model with finite element method was used to verify the results from the analytical model. Both of the analytical model and the numerical model reveal that the rock stress in the bottom hole rock can change from compressive stress in over-balanced conditions to tensile stress in underbalanced conditions, depending on the bottom hole pressure. A new finding from this study is that the temperature-induced stresses in the bottom hole rock can contribute significantly to the stress unbalance and cause rock failure. This paper provides drilling engineers insight and understanding of rock failure mechanics, which are useful for designing bottom hole pressure and gas injection rate to improve UBD performance.

A Study on the Mechanism of Bottom Hole Hydraulic Pulse Increasing Drilling Rate

Practically, the distribution and variation of the bottom-hole hydraulic energy has significant influence on drilling rate, and it’s helpful to induce bottom-hole hydraulic pulse to improve the rock breaking and drilling efficiency. The analysis of bottom-hole rock stress condition and cuttings start-up mechanism indicates that, bottom-hole hydraulic pulsation can decrease the bottom hole pressure, reduce the fracture strength of rock, and strengthen bottom-hole purification, thus improve the efficiency of rock breaking and drilling. The higher the pulsation value is, the more effective of the acceleration of the drilling rate, while well depth increases, the acceleration effect diminishes gradually. Simulation of jet flow field drilled by 3-nozzle 3-cone bit, combined with field application, verify the mechanism that bottom-hole hydraulic pulsation could improve ROP. The result of the study provides a basis for the development of practical technology.

Hydraulic Pulsed Cavitating Jet-Assisted Drilling

How to improve drilling rate in deep wells has been a hot subject. Based on modulating pulse jet and cavitating jet, a new drilling tool is designed which couples advantages of both pulse jet and cavitating jet. When drilling fluid flows through the tool during the drilling process, fluid is modulated to pulse and cavitate. Thus, pulse cavitating jet is formed at the outlet of the bit nozzle. Because of jet pulsation, cavitating erosion and local negative pressure affect bottomhole rock. Cleaning and breaking is enhanced and penetration speed is improved. Oil field tests in five wells show good applicability of the tool to bit types, formation, drilling densities, flow rates, and dynamic hydraulic drilling motors, etc. As a result, penetration rates are improved ranging from 10.1 to 31.5%; the maximum working time is about 235.5 hr in downhole. Pulse and cavitating jet coupling will afford an effective means to improve drilling rate for deep wells.

人工合成ダイヤモンド製細棒植込み型ビットの基本的性能に関する研究

New type of polycrystalline-diamond-compact (PDC) bits are tested for drilling hard rocks in laboratory conditions. Slender cylindrical inserts are used as cutters, so the bits are called as polycrystalline-diamond-insert (PDI) bits in this study. The inserts are 3-3.5mm in diameter, and 5mm in length in the test bit of 6cm in diameter. The inserts are composed of PDC and tungsten carbide (WC). Four types of inserts are tested: PDC sheathed by WC coaxially, PDC coverd by WC eccentrically, semicircular PDC welded with semicircular WC, and circular PDC.Drilling rate and bit life are measured of nine test bits with different profiles. The bit profile is designed considering inserts distribution pattern, cuttings removal, stable motion of bit, breakage of a center plug of bottom-hole rock and so on. No. 8 bit drilled about 30m of granite, of which compressive strength ranges from 130MPa to 160MPa, with drilling rate of about 2.2m/h at bit weight of 25-15kN and rotating speed of 99rpm. No. 9bit shows high drilling ability of 4m/h in granite, of which compressive strength is about 130MPa, at bit weight of 12kN and rotating speed of 99rpm. The bit has stable drilling rate and torque during long periods, when an adequate combination of bit weight and rotating speed is applied. Low bit weight is apt to wear off inserts faster than matrix, and high bit weight is apt to wear off matrix. At optimum conditions, inserts and matrix seem to wear off slowly keeping the relative height of inserts from the matrix.