Formation damage caused by drilling-fluid invasion, production, or injection can lead to positive skin factors and affect fluid flow by reducing permeability. When mud filtrate invades the formation surrounding a borehole, it will generally remain in the formation even after the well is cased and perforated. This mud filtrate in the formation reduces the effective permeability to hydrocarbons near the wellbore. It may also cause clays in the formation to swell, reducing the absolute permeability of the formation. In addition, solid particles from the mud may enter the formation and reduce permeability at the formation face.

ABSTRACT We developed a coupled THMC simulator employing fracture generation and a mineral dissolution/precipitation scheme for generated fractures based on the pH value of groundwater by improving our previously presented simulator, IPSACC. Then, the developed simulator was applied to evaluate the long-term evolution of rock permeability within a geological disposal facility of HLW under subsurface conditions considering the inflow of the alkaline cement solution from an artificial barrier. The predicted results indicated that the permeability of a damaged zone would eventually decrease to a value close to that of an undamaged zone due to the pressure solution at the contacting asperities within the fractures enhanced by an increase in the pH value due to the inflow of the alkaline cement solution from the artificial barrier. From these results, it is suggested that the impact of the pH alteration, brought about by the inflow of the alkaline cement solution, should be considered accurately for estimating a more realistic scenario of rock permeability evolution. 1. INTRODUCTION In order to investigate the performance of a geological repository of high-level radioactive waste (HLW) for delaying the transport of radionuclides, it is essential to predict the long-term permeability evolution of fractured rocks that work as a natural barrier under the coupled thermo-hydro-mechanical-chemical (THMC) conditions. Specifically, in the near field of a repository, the complex physical/chemical phenomena interact with each other by means of heat transfer from the waste body, the mass transport by groundwater, fracture generation, and geochemical reactions between the rock minerals and the groundwater. In particular, among the coupled phenomena, the occurrence of the geochemical reactions of the free-face dissolution/precipitation and pressure solution within the rock fractures generated during the cavity excavation may have a non-negligible impact on the long-term evolution of rock permeability. They depend on the chemical condition of the groundwater (e.g., pH value), and thus, should be influenced by the inflow of the alkaline cement solution from an artificial barrier in the geological repository of the HLW. Although many coupled numerical models have previously been proposed [1–3], coupled THMC simulations that consider the long-term permeability evolution of fractured rocks due to geochemical reactions with an alteration in the chemical condition of the groundwater, such as the pH value, have not been performed well. For example, although Liu et al. [1] presented a coupled THMC model employing the rock damage enhanced by hydraulic-chemical erosion and depending on the pH values, the pressure solution was not described. Ogata et al. [3] developed a coupled THMC simulator, IPSACC, that can describe the changes in rock permeability through fracture generation and subsequent sealing by the pressure solution and free-face precipitation. However, this simulator does not take into account the influence of the pH values of the groundwater.

ABSTRACT The process of polycrystalline diamond compact (PDC) cutter penetrating into the rock is important for the increase of cutting depth. However, this process is ignored in the past because the cutting is the main way to break the rock for the PDC bit. In this work, laboratory experiments were conducted to investigate this process. The variation trend of penetration force and the number of AE hit counts versus penetration depth was recorded in our tests. After the experiments, the shape of the damaged zone was analyzed. The results show that when the back rake angles of the PDC cutter is smaller than 20 degrees, the ductile failure is the main way to break the rock. With the increase of back rake angles, the brittle failure and ductile failure occurs alternately. The number of AE hit counts has a clear increase trend corresponding to the decrease of penetration force, which indicates the brittle failure dominates at this moment. According to the shape of the damaged zone, the increase of back rake angles will lead to the crack propagation in the process of PDC cutter penetration. Moreover, the cracks propagate more easily along the side where the PDC cutter has a larger angle with the rock surface. The main findings of this paper are helpful to understand the penetration process of the PDC cutter and increase the cutting depth. 1. INTRODUCTION The polycrystalline diamond compact (PDC) bit has been widely used in the oilfield since its introduction in the mid-1970s (Ersoy and Waller, 1997; Warren and Sinor, 1994). A series of single cutter tests and theoretical analyses were conducted to investigate the mechanism of rock breaking. On the basis of this, the performance of PDC bit has been improved a lot (Che et al., 2017; Chen et al., 2018; Cheng et al., 2019). However, the vast majority of these studies only focused on the cutting process, which is the main way to break the rock. The cutting depth remained constant and the process of PDC cutters indention was ignored in these experiments (Cheng et al., 2018). In fact, the indentation of PDC cutters is an important process for rock breaking. On the one hand, the rock failure mode will change from ductile to brittle with the increase of cutting depth. As a result, the improvement of indentation helps to enhance the efficiency of PDC bit (He and Xu, 2016; He et al., 2017; Liu et al., 2018; Zhou and Lin, 2013; Zhou and Lin, 2014). On the other hand, the indentation process will lead to the damage of rock as well, which is beneficial to the cutting process. However, to the best of our knowledge, there is no published literature that investigates the effects of PDC cutter indentation on the rock.

ABSTRACT This paper is aimed at understanding the interactions between a tunnel and the excavated damage zone (EDZ) under multiple hazard scenarios. The evolution of EDZ caused by different tunneling methods has been widely researched through physical and numerical modeling. Specifically, previous research has focused on quantifying stresses and deformations in shallow tunnels surrounded by soft soils under hazard loading scenarios. A truly comprehensive model considering hazard loading given an already damaged tunnel – incorporating key parameters including situ stress regime; transient unloading conditions subsequent to excavation; and anisotropic fracture networks and features such as faults and folds – is not currently available for hard rock and anisotropic stress fields. The overarching goal of this study is to quantitatively determine the response of a damaged lined tunnel post-excavation subjected to different hazards in a transversely anisotropic rock medium in terms of stresses and deformations. A rigorous finite difference based numerical model is developed. A tunnel embedded in relatively intact, massive, and hard rock, and subjected to high global initial in situ stresses is assessed. Various fracture network and sets are introduced surrounding the tunnel. The tunnel is then subjected to various fire hazard scenarios. 1. INTRODUCTION The response of a tunnel and tunnel lining in terms of stresses and displacements is of interest for design or rehabilitation purposes. Identification of the critical areas of a tunnel where damage is prone to occur –due to in situ stress conditions, excavation process, and/or external factors such as hazards or external loads – is crucial to design safe and effective excavation procedures, to plan efficient monitoring and retrofit methods, and to ensure long-term tunnel stability. To date, numerous studies have focused on assessing tunnel response under one extreme condition; however, no comprehensive study exists where the multi-variable nature of the problem is evaluated. The majority of current models developed to assess tunnel behavior under various in situ conditions have been developed assuming isotropic conditions within the tunnel plane. Furthermore, relevant research on failure mechanisms, and the proposed methods for mitigation and resiliency in tunnels implicitly include the aforementioned assumption. The anisotropic nature of the rock mass surrounding tunnels, especially in sedimentary rocks and rocks exhibiting layering or foliation such as gneiss, could notably impact the tunnel response(Hoek,1964 and Dammyr, 2016). Previous studies on rock mass anisotropy have identified two different failure mechanisms in such formations:(1) long-term shear-based deformation from squeezing of the rock mass surrounding the tunnel periphery; and (2) brittle failure mechanisms such as spalling and rock bursts (Singh, et al., 2006 and Dammyr, 2016). Multiple parameters have been identified to govern the dominant failure mechanism, and the location/extent of failure in tunnels: excess pore pressure from the depth of embedment with respect to the water table (Bobet, 2010); foliation (Dammyr, 2016); in situ horizontal-to-vertical stress ratio, k (Hijazo, et al., 2012);and geologic factors such as faults and folds with contact zone anisotropies (Lei, et al., 2017). Multiple methods have been developed to analyze and quantify stresses and displacements surrounding tunnels, through either utilizing a critical strain parameter (Singh, et al., 2006), or implementing closed form solutions (Bobet, 2010), or numerical modeling schemes such as hybrid finitediscrete element method (Lei, et. al, 2007).

Minglu Wu and Jiamin Zhu*, Key Laboratory of Unconventional Oil & Gas Development, China University of Petroleum (East China), Ministry of Education; Longlong Li, Hamad Bin Khalifa University; and Pengguang Li, Key Laboratory of Unconventional Oil & Gas Development, China University of Petroleum (East China), Ministry of Education Summary This paper presents the productivity formulas of a perforated vertical well on the basis of the dual-radial-flow model and then presents the productivity formulas of horizontal wells on the basis of disturbing elliptical flow theory. The threshold pressure gradient (TPG) is used to characterize the seepage characteristics of low-permeability reservoirs. By combing the above formulas, the authors proposed the productivity formulas of perforated horizontal wells in a low-permeability reservoir on the basis of the triple-radial-flow model. The formulas are derived considering the cases of the damaged zone being penetrated and being partially penetrated. At the same time, the flow patterns of single-phase flow and oil/water two-phase flows are considered. Sensitivity analysis on the performance of the perforated well shows that the productivity increases with the increase of the length of the horizontal well, penetration depth, shot density, perforation diameter, and phasing, and it decreases with the increase of TPG, compaction thickness, and crush degree of the crushed zone; the influence of the length of the horizontal well, TPG, penetration depth, shot density, compaction degree, phasing, perforation diameter, and compaction thickness on the productivity is in descending order. Introduction As the main completion method in the petroleum industry and accounting for more than 90% of total completions used in oil/gas production wells and injection wells, perforation completion is also widely used in low-permeability reservoirs.

Abstract Compressibility needs to be accounted for when estimating productivity decline in closed gas and oil reservoirs, and in closed aquifers. Previous works derived an analytical model and well index for inflow performance accompanied by fines migration and consequent permeability damage for incompressible flow towards well. In the present work, we account for fluid and rock compressibility. The problem with given and constant well production rate is investigated. Mathematical model is developed, which provides well productivity index decline with time. Under this model, the solution of damage-free compressible flow in a closed reservoir is matched with the impedance growth formulae for incompressible flow in the well vicinity. The well production data have been successfully matched by the model; the tuning parameters have the common values. It allows indicating the fines mobilization, migration and straining as possible well impairment mechanism in wells under investigation.

Abstract Understanding the mechanisms of fluid flow in unconventional shale reservoirs is of great interest as these mechanisms have significant impacts on long term economic development of such reservoirs. In shale rocks, the average size of pore/throat is much smaller than the average pore size in conventional rocks which results in higher capillary pressures. Such high capillary pressures can strongly influence the two-phase flow specifically around the wellbore by preventing the fluid flow from matrix to hydraulic fractures and resulting in liquid holdup. In addition, the pronounced poroelastic properties of shale matrix make the flow properties to be extremely sensitive to the effective stress. As a result, the production rate and well deliverability of shale reservoirs can be severely affected by stress dependent capillary pressure around the wellbore area while this impact has not been investigated yet. In this study, a numerical approach was adapted to solve the analytical formulation of two-phase flow considering the capillary and viscous forces in matrix around hydraulic fractures. To evaluate the integrity of the simulation results, the predicted liquid saturation profiles were compared with some experimental data reported in the literature where liquid saturation profiles were measured by CT scanner under both viscous and capillary dominated flow conditions. Then, the two phase flow in tight formations were simulated and the obtained liquid saturation profiles were used to estimate the equivalent relative permeability curves at different stress conditions. The results showed that the liquid holdup in the matrix around the hydraulic fractures can be accumulated even up to a meter that significantly reduces the relative permeability values in this zone. This liquid holdup (or capillary end effects) depends on several parameters including effective stress applied to the formations. In addition, the effects of viscous forces on liquid holdup were investigated in terms of fluid velocity. It is found that, a higher fluid velocity(or flow rate) which can be achieved by increasing drawdown pressure (reduction of bottom hole pressure) can cause a significant damage to the matrix permeability around the hydraulic fractures. This damage also adversely affects flow and can promote the capillary dominated flow. The results of this study improve our understanding of flow mechanisms in unconventional reservoir rocks. This knowledge is required for shale reservoir simulation and cost effective production from hydraulic fractured wells in shale reservoirs.

Abstract Temperature transient analysis is evolving to implement the production induced thermal perturbations for reservoir and near wellbore characterization purposes. To observe strong temperature signals, temperature well testing often involve high drawdown, which may induce non-Darcy flow effect in the near wellbore region. The existing analytical solutions for temperature transient analysis generally assume Darcy’s law for pressure profiles, which can be invalid in both gas and oil productions. In this study, we derive an analytical solution for temperature transient analysis of slightly compressible fluid producing from a vertical well considering the non-Darcy flow effect. To validate the developed analytical solution, we model the sandface temperature signals analytically and verify the results with numerical simulation conducted under comparable conditions in non-damaged and damaged reservoirs. Good agreements are achieved between analytical and numerical modeling results under different production rates and non-Darcy flow coefficients. The non-Darcy flow effect significantly increases the sandface temperature variations during production, which approach to a constant increment for sufficient long production time. We develop a new permeability estimation method considering the non-Darcy flow effect, which is also applicable for damaged zone property and non-Darcy flow coefficient estimations. Two criteria to apply temperature transient analysis for non-Darcy flow effect evaluation are revealed as critical Forchheimer number and accuracy of the downhole temperature monitoring system. Sensitivity analyses reveal that the non-Darcy flow coefficient impacts the magnitude of the sandface temperature signals while the production rate affects the slope of temperature profiles in a semi-log plot. Based on the findings in this work, we build on the existing characterization procedures for temperature transient analysis and incorporate the non-Darcy flow effect to estimate the permeability and non-Darcy flow coefficient. For the cases presented in this study, the inversion process of temperature transient analysis can accurately estimate the reservoir and damaged zone permeabilities, as well as damaged zone radius (less than 10% errors). We also evaluate the non-Darcy flow coefficients with acceptable accuracies (less than 30% errors) in a field scale. With these improvements, the applicability of temperature transient analysis using analytical solutions can be extended from cases with limited sandface temperature signals of a few degC to stronger signals of 30-40 degC considering the non-Darcy flow effect.

Abstract The proppant embedment into rock during the operation of the hydraulically fractured well is one of the reasons for the production rate decrease. Most researchers analyze this problem on the basis of laboratory experiments and do not take into account the well production data. The article proposes a mathematical model of proppant embedment, which makes it possible to estimate the change in the fracture conductivity. A joint calculation of the stress and pressure fields is carried out. The stress values are used to check the conditions at the start of proppant embedment. The discussed example shows a decrease in the width and conductivity of the fracture due to the proppant embedment, and the conditions are found under which this effect is significant. Usually, in order to assess the fracture conductivity, well tests are carried out. Using proposed mathematical model together with the well test results, makes it possible to estimate the contribution of proppant embedment to the change in fracture conductivity, as well as to identify additional factors leading to well production rate decrease.

Abstract Productions from oil and gas reservoirs can induce significant pressure and temperature changes at the wellbore. The temperature signal is sensitive to reservoir properties and production parameters which can be very useful in characterizing the reservoir. In this work, we introduce novel analytical solutions to determine the temperature signal associated with theproduction of slightly-compressible hydrocarbon from a vertical well, and apply the solutions to the production from oil and gas reservoirs. Our procedures to obtain the analytical solutions from the governing equation involve making relevant assumptions that allow rigorous solutions to be constructed using Laplace transform. We extend the analytical solutions to include the near-wellbore damage, and to characterize the damaged zone. Our results of the analytical models are benchmarked with those from a commercial numerical simulation software. We substantiate that the Joule-Thomson effect on the temperature profile is significant in near-wellbore region, and adiabatic expansion effect extends the radius of investigation of the transient temperature signal. The damaged zoneanalytical solution shows that damaged zone radius and permeability separately affect thetemperature transient signal. This isunlike the pressure transient response for which the effect of damage zone properties is lumped into a single parameter, i.e. the skin factor. The analytically derived equations for slopes of Joule-Thomson and adiabatic expansion effects in undamaged and damaged reservoir present very close agreement with those obtained numerically. We provide semi-log temperature interpretation techniques to determine the reservoir permeability and porosity, and damaged zone radius and permeability.