Li, Jun (China University of Petroleum-Beijing) | Liu, Yuetian (China University of Petroleum-Beijing) | Xue, Liang (China University of Petroleum-Beijing) | Cheng, Ziyan (China Petroleum & Chemical Corp SINOPEC) | Kong, Xiangming (China University of Petroleum-Beijing) | Li, Songqi (China University of Petroleum-Beijing)
After fracture treatment in unconventional reservoirs, the in-situ stress and fluid pressure are greatly changed in the reservoir because of the generation of fracture networks. In order to get high production, efforts are made to get close fracture spacing and long fracture length in-situ field, which in turn make fracture distribution become complicated as the range of fractures size and density is widespread.
In this work, the finite element method is used to analysis flowback around hydraulic fracture among complex fractures networks, which consider the coupled effects of flow and geomechanics.
The reservoir is assumed to be a 3-D poroelastic medium. According to the fracture sizes, the fracture is divided into three types. These small natural fractures are treated as SRV regions, hydraulic fractures, natural fracture in middle and large sizes are explicitly represented using LGR. Finite element method simulates fracture deformation and the two-phase fluid flow in the reservoir during flowback stage. The physical properties are altered by the coupled flow and geomechanics in the reservoir.
The fluid pressure, stress and flowback production over time around these fractures are recorded. The results show that during the flowback period, the production experience a sharp decrease. The porosity and permeability in the reservoir are greatly reduced because of the coupled effects. These explicit natural fractures influence the hydraulic fractures. As the hydraulic fracture spacing reduced, the stress shadow effects become more serious and the flowback production decreases.
This work helps understand the flowback analysis with coupled geomechanics and flow effects in the complex fracture networks in the unconventional reservoirs and physical properties effects in different reservoir conditions.
Onur, Mustafa (The University of Tulsa) | Galvao, Mauricio (Petrobras) | Bircan, Davut Erdem (The University of Tulsa) | Carvalho, Marcio (Pontifical Catholic University of Rio de Janeiro) | Barreto, Abelardo (Pontifical Catholic University of Rio de Janeiro)
The objectives of this study are to (i) provide analytical transient coupled wellbore/reservoir model to interpret/analyze transient temperature drawdown/buildup data acquired at both the producing horizon (sandface) and a gauge depth above the producing horizon (wellbore) and (ii) delineate the information content of both transient sandface and wellbore temperature measurements. The analytical models consider flow of a slightly compressible, single-phase fluid in a homogeneous infinite-acting reservoir system and provide temperature-transient data for drawdown and buildup tests produced at constant rate at any gauge location along the wellbore including the sandface. The production in the wellbore is assumed to be from inside the production casing. The models account for Joule-Thomson (J-T), adiabatic fluidexpansion, conduction and convection effects as well as nearby wellbore damage effects. The well/reservoir system considered is a fully penetrating vertical well in a two-zone radial composite reservoir system. The inner zone may represent a damaged (skin) zone, and the outer (non-skin) zone represents an infinitely extended reservoir. The analytical solutions for the sandface transient temperatures are obtained by solving the decoupled isothermal (pressure) diffusivity and temperature differential equations for the inner and outer zones with the Boltzmann transformation, and the coupled wellbore differential equation is solved by Laplace transformation. The developed solution compares well with the results of a rigorous thermal numerical simulator and determines the information content of the sandface and wellbore temperature data including skin zone effects. The analytical models can be used as forward models for estimating the parameters of interest by nonlinear regression built on any gradient-based estimation method such as the maximum likelihood estimation (MLE).
A challenge in oil-reservoir studies is evaluating the ability of geomechanical, statistical, and geophysical methods to predict discrete geological features. This problem arises frequently with fracture corridors, which are discrete, tabular subvertical fracture clusters. Fracture corridors can be inferred from well data such as horizontal-borehole-image logs. Unfortunately, well data, and especially borehole image logs, are sparse, and predictive methods are needed to fill in the gap between wells. One way to evaluate such methods is to compare predicted and inferred fracture corridors statistically, using chi-squared and contingency tables.
In this article, we propose a modified contingency table to validate fracture-corridor-prediction techniques. We introduce two important modifications to capture special aspects of fracture corridors. The first modification is the incorporation of exclusion zones where no fracture corridors can exist, and the second modification is taking into consideration the fuzzy nature of fracture-corridor indicators from wells such as circulation losses. An indicator is fuzzy when it has more than one possible interpretation. The reliability of an indicator is the probability that it correctly suggests a fracture corridor. The indicators with reliability of unity are hard indicators, and “soft” and “fuzzy” indicators are those with reliability that is less than unity.
A structural grid is overlaid on the reservoir top in an oil field. Each cell of the grid is examined for the presence and reliability of inferred fracture corridors and exclusion zones and the confidence level of predicted fracture corridors. The results are summarized in a contingency table and are used to calculate chi-squared and conditional probability of having an actual fracture corridor given a predicted fracture corridor.
Three actual case studies are included to demonstrate how single or joint predictive methods can be statistically evaluated and how conditional probabilities are calculated using the modified contingency tables. The first example tests seismic faults as indicators of fracture corridors. The other examples test fracture corridors predicted by a simple geomechanical method.
Accurate and robust well modeling is essential for performing reservoir simulations of practical interest. The Multi-Segment well (MSWell) model is able to describe the well topology and accurately represent the multiphase multicomponent flow and transport behavior in the wellbore. The fully coupled method (FC) has been developed and widely applied on coupled reservoir and MSWell modeling due to its unconditional stability and consistent implementation. A local well solver can be applied to provide a better nonlinear precondition for MSWell variables in order to accelerate the nonlinear convergence of the FC method.
However, solving the coupled MSWell and reservoir model in a fully implicit scheme can still present limitations on some practical applications. First, the well or surface facility solver can be separate from the existing reservoir simulator, making it challenging to employ the fully implicit method. Second, complex linear and nonlinear solvers need to be designed to pair the specific wells and reservoir models. These solvers have to account for the different flow characteristics and discretization domains between reservoir and MSWell. A sequential coupling scheme can become preferable in such situations.
Sequential fully Implicit method (SFI) splits the fully coupled reservoir and MSWell equations into two parts and solves them sequentially. In spite of accomplishing an implicit coupling in a sequential scheme, SFI suffers the slow outer loop convergence rate especially when reservoir is strongly coupled with the wells, which is very often the case. The slow convergence is caused by the linear convergence rate of the fix point iteration used in the SFI. Here, we developed a sequential implicit Newton's method (SIN) for coupled MSWells. SIN incorporates a Newton update at the end of each sequential step to achieve a quadratic convergence of outer iterations, while require a limited extra computational cost. Numerical results show that SIN attains comparable nonlinear Newton iterations with the FC in the coupled heterogeneous reservoir and complex MSWell problems.
Liu, Hui (University of Calgary) | Chen, Zhangxin (University of Calgary) | Shen, Lihua (University of Calgary) | Zhong, He (University of Calgary) | Liu, Huaqing (AMSS, Chinese Academy of Sciences) | Yang, Bo (University of Calgary) | Ji, Dongqi (University of Calgary) | Zhu, Zhouyuan (China University of Petroleum) | Zhan, Jie (Xi'an Shiyou University)
This paper deals with the development of our parallel reservoir simulator that is designed for giant reservoir models. It considers oil, water and polymer, and a reservoir can be a conventional reservoir without fractures or a naturally fractured reservoir. For polymer flooding, the simulator can model polymer retention, adsorption, an aqueous phase permeability reduction and viscosity increase, and an inaccessible pore volume. Here fractures are modeled by the dual porosity and dual permeability method. The finite difference (volume) method is applied to discretize the model, upstream techniques are employed to deal with rockfluid properties, and the fully implicit method in time is applied. The linear systems from the Newton method are ill-conditioned and a scalable CPRtype preconditioner is employed to accelerate the solution of these linear systems. The computed results are compared with those from commercial simulators, and they match very well.
The present work focuses on the stability of drill strings in vertical wellbores. The first rigorous treatment of stability of drill strings for vertical wellbores was presented by Lubinski (1950) and his equation is till most widely used in the industry. Cunha (2004) stated that since Lubinski (1950) used power series to solve differential equation governing the stability problem, and the terms of power series become very large for long drill strings, therefore, after a certain length, the calculations may lead to inaccurate results. Mitchell and Miska (2011) stated that analytical solution for infinite-length drill string is used for deep vertical wells in the industry. The subject studied in this study is of great importance in designing the bottom hole assemblies in deep and ultra-deep vertical wells to eliminate problems associated with instability of drill strings. The study includes Finite Element Method (FEM) solution of critical sinusoidal buckling force for 5 different drill collars with 21 different lengths starting from 1000 ft. up to 25000 ft. The main objectives of the study are to see the difference between post-buckling behavior of slender-dominated long hanging drill strings with stiffness-dominated short hanging drill strings in vertical wellbores; to investigate the effect of flexural rigidity of drill collars on decrease of the critical buckling forces as a length parameter of the drill string; and to see the behavior and amount of decrease in the critical buckling forces as the length of the drill string increases. And, it is showed that critical buckling force decreases as the depth of the well increases according to FEM solutions, although, analytical solution gives only a fixed critical buckling force for a specific pipe independent from the length. Also, it is showed that post-buckling behavior of slender-dominated long hanging drill strings with stiffness-dominated short hanging drill strings in vertical wellbores are different.
Laser perforating is a new approach to the generation of uniform holes in oil and gas reservoir wells at a selected pitch to improve the permeability of rocks. Laser drilling in rocks is a very complex phenomenon that its performance depends on many factors. Since it is not possible to consider all of these factors in the laboratory, numerical modelling is used. In this study, a finite element code (FEM) has been taken to model the thermal and mechanical stresses induced by ND: YAG laser drilling in the hydrocarbon reservoir rock samples. For this purpose, the software ABAQUS was used to analyze the thermal and mechanical stresses induced by laser. It is found that Numerical models show good agreement with the actual observation of holes drilled by the laser. During laser drilling, the rock temperature quickly increases in a few seconds and immediately reduces, thus instantaneous heating and cooling process cause thermal stresses around the hole. Also, the maximum value of thermo-mechanical stress exceeds the strength of the limestone rock and consequently, the formation of cracks and fractures in the wall of the hole are unavoidable.
Laser perforating is a new scientific way to the creation of uniform holes in petroleum reservoir wells to increase the permeability of rocks (Ahmadi et al., 2011). Thermal stress generated by differential thermal expansion of minerals and high-temperature gradient, cause to break the bonds between the grains. In this range of temperature, physical and chemical changes occur that are associated with the process of spallation. A primary physical change associated with this process is due to the thermal expansion of the grains of the rock. For example, the expansion of quartz and plagioclase grains in sandstone lead to a sudden temperature increase in it (Gahan et al., 2004).
As closely-packed grains in the matrix expand with a rapid rise in temperature, they develop thermal stress fractures and cracks within the grains, as well as break the cementation of adjacent grains. As a result, an affected grain will begin to break free from one another (Salehi et al., 2007). Laser effects appear in two steps in rocks, firstly, the creation of a hole in the rock and secondary include melting, evaporation, laser beam gases and micro fractures.
Zhang, Bo (College of Geo-exploration Sciences and Technology, Jilin University, Changchun, Jilin, 130021, China) | Yin, Changchun (College of Geo-exploration Sciences and Technology, Jilin University, Changchun, Jilin, 130021, China) | Liu, Yunhe (College of Geo-exploration Sciences and Technology, Jilin University, Changchun, Jilin, 130021, China) | Ren, Xiuyan (College of Geo-exploration Sciences and Technology, Jilin University, Changchun, Jilin, 130021, China) | Huang, Xin (College of Geo-exploration Sciences and Technology, Jilin University, Changchun, Jilin, 130021, China) | Cai, Jing (College of Geo-exploration Sciences and Technology, Jilin University, Changchun, Jilin, 130021, China) | Wang, Cong (College of Geo-exploration Sciences and Technology, Jilin University, Changchun, Jilin, 130021, China) | Weng, Aihua (College of Geo-exploration Sciences and Technology, Jilin University, Changchun, Jilin, 130021, China)
Adaptive mesh refinement has proved to be very effective tool in increasing the accuracy of 3D modeling by many scholars. However, most researches focused on the frequency-domain forward modeling problem, not much attention has been paid to the adaptive method for timedomain EM forward modeling. In this paper, we create an adaptive finite-element scheme for 3D time-domain airborne EM forward modeling. To discretize the timedomain Maxwell equation, we use directly the backward Euler method. To effectively execute the adaptive process, we estimate the posterior error based on the continuity condition of the normal current. The accuracy check against one-dimensional model and analysis on model responses confirm the effectiveness of our algorithm.
Presentation Date: Wednesday, October 17, 2018
Start Time: 1:50:00 PM
Location: Poster Station 4
Presentation Type: Poster
Reverse time migration (RTM), a wave equation based imaging tool, is widely used due to its ability to recover complex geological structures. However, RTM also has a drawback in that it requires significant computational cost. To reduce this cost, we utilize a frequency-adaptive multiscale spatial grid to enhance the efficiency of the wave simulations. We also apply the generalized multiscale finite element method, which solves local spectral problems on a fine grid to incorporate the influence of fine-scale heterogeneity in basis functions used for rapid, coarse scale modeling. We further enhance the speed of computation without sacrificing accuracy by utilizing coarser grids for lower frequency waves, while applying finer grids for higher frequency waves. In the proposed method, we can control the size of coarse grid and number of eigen-frequencies to tune the tradeoff between speedup and accuracy. Although wave solutions are computed on the coarser grid, we can still obtain the RTM images without reducing the resolution by projecting coarse wave solutions to the fine grid.
Presentation Date: Wednesday, October 17, 2018
Start Time: 1:50:00 PM
Location: Poster Station 20
Presentation Type: Poster
We formulate and apply a finite element method for blind deconvolution with time-variant wavelets. This approach allows for wavelets whose period or frequency evolves, and involves no use of explicit smoothing or sparseness constraints. Rather, desired wavelet and reflectivity properties are imposed through the choice of basis functions. Examples on synthetic and field data sets demonstrate the ability of the method to simultaneously solve for reflectivity and a dynamic, variable frequency wavelet.
Presentation Date: Tuesday, October 16, 2018
Start Time: 1:50:00 PM
Location: 211A (Anaheim Convention Center)
Presentation Type: Oral