Harris, J.G. and G.I. Block, 2005.
Source: Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 46: 3765 – 3783.

Abstract

A layer of homogeneous, isotropic, elastic material overlays a substrate of similar material. The shear wavespeed within the layer is less than that of the substrate causing waves to be trapped within the layer. At the interface a long inclusion, that grows gradually until it reaches a constant thickness, is introduced. The inclusion is composed of a material whose shear wavespeed is less than that in the layer; it is described as slow. It is imagined that the lowest surface-wave mode of the structure is incident to the growing inclusion. Numerical calculations show that the growth of the slow inclusion brings the wavenumber of this lowest mode into an interval where it is close to that of the second mode, thus exciting it. This process is repeated when the wavenumber of the second mode is brought close to that of the third. Within these intervals, energy is exchanged among the coupling modes. Outside of these localized intervals, the modes propagate independently of one another and their amplitudes vary such that the flux of energy in each mode is conserved; they are said to propagate adiabatically. Reflections are also excited, but are shown to be very small in magnitude.

Presented at SPE Asia Pacific Health, Safety and Environment Conference and Exhibition, Kuala Lumpur, Malaysia, September 2005.

Abstract

Production optimisation and improving a project’s net present value (NPV) for global hydrocarbon producers need strategies for produced water management (PWM), in order to eliminate significant economic and environmental barriers. PWM issues hamper production by restricting additional development or adding costs (US$0.15 to US$2.50/barrel of oil (BoO)). Operators raise the economic limit for well operability or abandon existing wells, while substantial recoverable reserves remain in situ. PWM poses the biggest challenge yet offers considerable benefits to brownfield operators. While operators around the globe experience identical problems, local conditions and requirements dictate that solutions are region-specific. Regions can vary significantly and boundaries may be set geologically, geographically or politically. An obvious example is PMW in offshore deepwater conditions in contrast to onshore and/or the Arctic or other sensitive areas. PWM issues are multi-faceted. In many cases, the overall solution may require several separate steps for complete resolution (reduction, chemical removal, profile control, separation, treatment, disposal and waterflooding use, etc.). Hence, two dominating themes emerge from the stakeholders’ point of view the need for holistic PWM and the absence of ‘silver bullets’.

Presented at SPE Offshore Europe Oil and Gas Exhibition and Conference, Aberdeen, United Kingdom, September 2005.

Abstract

This paper presents a study in which injector performance is evaluated during water flooding operations in a North-Sea Field. It is well known that water quality and well conditions strongly affect injection performance. Poor water quality leads to plugging and bacterial growth which results in a loss of permeability and injectivity decline. The effect of poor water quality can be mitigated through the treatment and removal of harmful solids, dissolved oxygen (DO) and bisulfites (BI). Long-term remideation can be achieved through various stimulation techniques. However, technology limits and frequent treatment plant upsets can negate the effects of these mitigations and frequent stimulations would result in significant costs. Optimization of injection operations depends on the selection of the proper strategy and answering questions such as “Which water contaminants have the biggest impact?” and “How much would stimulation improve injectivity?” Surprisingly, reliable answers are difficult to obtain because interactions among injection variables are obscured by highly-complex, process dynamics (time-dependencies).In this study, data mining techniques were applied in order to understand factors that affect field injection performance.The study used 11 years of injection and water quality data from 14 wells in two different blocks. Dynamic models of well behavior were synthesized using a combination of “artificial neural networks” (ANN), a machine learning technique from the field of Artificial Intelligence and “multivariate state space reconstruction” from the Chaos Theory. Sensitivity analysis performed using the ANNs revealed the relative impacts of each variable on injector performance. Despite different histories for each well, the models were relatively uniform in quantifying the relationships between the variables.Findings included – a different response to acidizing in each block; delayed cumulative formation damage from each variable of 1–3 months; the impact of DO was more than twice that of particulate matter (PM)and BI; and clorides (CL) had only a small affect on suppressing bioactivity. The uniformity and clarity of these results reduce uncertainties and provide operators with detailed knowledge to help optimize their injection designs.

Geo-mechanics of Batch Injection in Horizontal Disposal Wells – North Sea

Zaki, K.; Kristiansen, T. G.; Abou-Sayed, A. S.; Summers, C. W.; Wang, G. G.; and Sarfare, M.D.
Alaska Rocks 2005, The 40th U.S. Symposium on Rock Mechanics (USRMS), Anchorage, Alaska, June 2005.
https://onepetro.org/ARMAUSRMS/proceedings-abstract/ARMA05/All-ARMA05/ARMA-05-672/117722

Abstract

The disposal domain concept is arguably accepted as the prevalent storage mechanism during the batch injection of drill cuttings. The disposal domain is best thought of as an elliptical realm surrounding the well. This phenomenon is well documented by field and laboratory observations and has been addressed in previous work. Most studies and field operations have centered around vertical or near vertical wells. Rarely have horizontal wells been designed for conversion to injectors or used for disposal and therefore, the effect of batch injection and the created disposal domain have not been addressed. The current paper will provide an integrated look at the fracturing process that occurred in a horizontal disposal well. The well is located in the Valhall Field, North Sea, offshore Norway. The well has multiple perforated intervals. The work will address a modified disposal domain concept as it applies to horizontal wells. The paper illustrates a history of the fracture nature (geometry and extent) and propagation across the various layers. Comparison of the field pressure history and the simulation results will be addressed. Further studies to address the effect of a workover carried out in the well, plug placement, are currently under way. These modifications will be addressed in a future paper.

A Geomechanics Updates on Alaska’s GNI Project: Monitoring, Assessment, Validation and Assurance for the World Largest Drill Cuttings Injection Project

Abou-Sayed, A.S. and Wang, G.G. Advantek International, Engle, H. BP Alaska, Willson, S.M. BP, Bill, M. ASRC Energy SErvices
40th U.S. Symposium on Rock Mechanics (USRMS) Rock Mechanics for Energy, Mineral and Infrastructure Development in the Northern Region, June 25-29, 2005, Anchorage, Alaska
https://onepetro.org/ARMAUSRMS/proceedings-abstract/ARMA05/All-ARMA05/ARMA-05-676/117709

Abstract

The ongoing Prudhoe Bay Grind and Inject (GNI) program began in the mid 1990’s. State and Federal Environmental regulations require the operator to assure the injected material not be allowed to migrate to groundwater aquifer zones or to the surface. Sensitive fracture modeling and annual surveillance activities have been conducted to ensure the integrity of the process. A recent extensive technical review of the field’s collected information, injection simulation results and well testing data was conducted to evaluate the containment of the injected slurry. The current paper discusses the results of engineering simulation and field monitoring efforts within the perspective of a geomechanics review to verify the injected slurry extent. Novel interpretations of pressure records along with new simulation results are presented. Analysis of the waste domain simulation, based on fracture propagation in compactable (soft) rocks and well testing data, confirm that the fracture domain is confined in the intended injection zone. This paper will also compare the results of successive fracture domain simulations to the results of annual well tests over the last 5 years. Finally, the applicability of other potential means of disposal domain monitoring and diagnostic tools such as surface and downhole tiltmeters, micro seismic diagnostic, and tracers, are assessed from a geomechanics perspective.

Abou-Sayed, A.S., Zaki, K., Wang, G., Sarfare, M.
Presented at SPE European Formation Damage Conference, Sheveningen, The Netherlands, May 2005.

Abstract

The majority of injectors are likely to be fractured (intentionally or unintentionally) during their life cycles. Assessment of facture conductivity damage and associated surrounding formation damage is an essential step for maximizing and maintaining injector performance during produced water injection for water flooding. Fractured injectors experience less injectivity decline over time in comparison to matrix injectors. The current paper will present a mechanistic model for the plugged fracture behavior and discuss its applications to field cases. In addition, implications, recommendations and best practices for optimized produced water injection will be addressed within this context. The well undergoes two distinct processes that alternate over its life to create the sustained injectivity behavior associated with this scheme. Injectivity damage can occur particularly in pseudo-matrix conditions, where a stationary fracture progressively plugs (in addition to permeability impairment of the surrounding reservoir rock), resulting in pressure buildup and injectivity decline. As solids are deposited in the fracture, the “effective” exposed surface area is gradually reduced, at least partially, as a function of the injected amount of solids.Furthermore, fracture face damage (external filter cake build-up) and accumulated surrounding formation damage (internal filter cake build-up) progress as the volume of injected water increases. This plugging process continues until the injection pressure required for the designated injection rate reaches a critical value, which is greater than the pressure required to re-fracture the formation and/or to extend or propagate the plugged fracture. At such pressure a sudden fracture propagation or further breakdown of the formation is evidenced by a sudden increase in the Injectivity Index or similarly, a sudden drop in the Reciprocal Injectivity Index – RII – (unit injection pressure per unit constant injection rate).Thereafter, the required pumping pressure drops and/or the injection rate increases with the newly created surface area that has been caused by fracture propagation or initiation.The process then repeats itself. As the pressure rises it approaches and finally exceeds the fracture pressure required for further propagation, resulting in a saw-toothed shape pressure-time (or rate-time) behavior. The slopes of the bounding lines are controlled by the damaged formation and fracture characteristics, as well as the injected water quality.The slopes can be made to diverge, converge or remain almost parallel. The third option indicates a well optimized injection scheme. The final section of this paper will imply lessons learned, mitigation strategies, conclusions and recommendations based on analyses of field data from a number of produced water injectors. Pressure transient analysis and results from pressure fall-off test will be presented to verify the study results.

Block, G., 2005.
Source: In Proceedings of McMat2005: 2005 Joint ASME.ASCE/SES Conference on Mechanics and Materials, June 1 – 3, 2005, Baton Rouge, Louisiana.

Abstract

Electrokinetic (EK) phenomena in sediments arise from relative fluid motion in the pore space, which perturbs the electrostatic equilibrium of the double layer at the grain surface. We have developed EK techniques in the laboratory to monitor acoustic wave propagation in electrolyte-saturated, unconsolidated sediments. Our experimental results indicate that as an acoustic wave travels through electrolyte-saturated sand, it can generate electric potentials greater than 1 mV. A careful study of these potentials was performed using medium-grain sand and loose glass microspheres for a range of pore fluid salinities and ultrasonic frequencies. Experimental results are also shown to compare well with numerical and analytical modeling based on the coupled electrokinetic-Biot theory developed by Pride (1994).

Guo Yonggui and J.K. Morgan, 2004
Paper presented at Journal of geophysical research, 109, B12305, doi:10.1029/2004JB003044.

Abstract

Laboratory experiments of granular shear deformation demonstrate that loading conditions and grain characteristics can significantly affect the macroscopic friction of a granular material under shear. We have examined the variation of maximum sliding friction with normal stress and grain shape using a version of the distinct element method (DEM) that includes bonds between adjacent particles. In this way, arbitrarily shaped grains can be generated to reproduce more realistic fault gouge with a range of grain sizes and shapes. Two types of grains were designed to represent quartz gouge: rounded grains composed of seven close-packed particles and triangular grains composed of six close-packed particles. DEM experiments were conducted by shearing granular assemblages with different grain shape distributions using the identical boundary configurations (i.e., wall surface roughness) over a range of normal stresses from 5 to 100 MPa and were compared to equivalent experiments using reference circular particle assemblages. The results show an inverse power law relationship between normal stress and maximum sliding friction in all cases, where both its coefficient and exponent are dependent on gouge angularity. Under normal stress over 20 MPa, triangular grain assemblages exhibited the highest frictional strength and also the highest abundance of rotating grains, demonstrating that enhanced grain rolling alone does not explain the low frictional strength of simulated granular assemblages.

Morgan, J.K., C.J. Marone, Y. Guo, J. Anthony, and M. Knuth, 2004
Presented at 4th ACES Workshop Proceedings, edited by A. Donnelan, M. Matsu’ura, and P. Mora, APEC Cooperation for Earthquake Simulation.

Abstract

Laboratory studies of granular shear zones have provided significant insight into fault zone processes and the mechanics of earthquakes. The micromechanisms of granular deformation are more difficult to ascertain, but have been hypothesized based on known variations in boundary conditions, particle properties and geometries, and mechanical behavior. Numerical simulations using particle dynamics methods (PDM) can offer unique views into deforming granular shear zones, revealing the precise details of granular microstructures, particle interactions, and packings, which can be correlated with macroscopic mechanical behavior. Here, we describe a collaborative program of comparative laboratory and numerical experiments of granular shear using idealized materials, i.e., glass beads, glass rods or pasta, and angular sand. Both sets of experiments are carried out under similar initial and boundary conditions in a non-fracturing stress regime. Phenomenologically, the results of the two sets of experiments are very similar. Peak friction values vary as a function of particle dimensionality (1-D vs. 2-D vs. 3-D), particle angularity, particle size and size distributions, boundary roughness, and shear zone thickness. Fluctuations in shear strength during an experiment, i.e., stick-slip events, can be correlated with distinct changes in the nature, geometries, and durability of grain bridges that support the shear zone walls. Inclined grain bridges are observed to form, and to support increasing loads, during gradual increases in assemblage strength. Collapse of an individual grain bridge leads to distinct localization of strain, generating a rapidly propagating shear surface that cuts across multiple grain bridges, accounting for the sudden drop in strength. The distribution of particle sizes within an assemblage, along with boundary roughness and its periodicity, influence the rate of formation and dissipation of grain bridges, thereby controlling friction variations during shear.

Presented at SPE Annual Technical Conference and Exhibition, Houston, Texas, September 2004.

Abstract

The widespread use of FracPack technology in deepwater reservoir has been a growing practice. Its purpose is sand control and well stimulation. To-date, field applications and fracture treatments have been designed using traditional hydraulic fracturing simulators that apply LEFM theories. While this is adequate for hard rocks (e.g., tight gas formations), the fracture geometry predictions fall short when applied to fracturing soft rocks. Soft rocks are normally at incipient plasticity and, hence, are prone to compaction. Compaction, or plastic rock deformations during sand control FracPacks operations and disposal of drilling cuttings slurries in soft layers. The capacity of the created fracture to store or accept solids, the conditions of the rock strength near the fracture faces and the near well/fracture rock porosity or permeability are all highly impacted by the rock compaction during the fracture propagation process. The objective of the presented research is to assess the impact of compaction and plasticity on fracture geometry and formation properties around the fracture. In particular, it is important to quantify the details of the geometry of factures generated during FracPack and waste disposal operations as well as the porosity/permeability changes in the vicinity of the fracture faces. In the current paper, rock behavior is described by a variation of the Cam-Clay model. This model represents an inelastic, work hardening model that, depending on the loading path, could predict both compaction and dilatency, in a given formation. This is particularly useful in modeling soft or elasto-plastic compacting formations since the fracture propagation is heavily driven by the leak off into the formation and the in situ stress profile. Formation low permeability leads to lower leak off rates, especially if the injected slurry has a leak-off control additive. This scenario leads to compaction of the rock along the fracture sides. High permeability at the tip results in a large amount of fluid leak off into the formation causing the near-tip zone to dilate during slurry injection and fracture propagation. The current paper presents results of fracture simulation in compacting rocks including fracture geometry, fracturing pressure and porosity/permeability alteration around the fracture. In the previous paper, results of finite element model provided a benchmark to simulation results. The present work, on the other hand, shows the extent of formation disturbance and porosity/permeability alteration, as well as propped fracture characterization in FracPacks. Finally, the model results addressed the disparity between conventional FracPacks designs and actual treatment data. The observations confirm the need for careful consideration of rock plasticity in fracture simulation to avoid FracPack failures and minimize the absence of TSO response in some field implementations.

Presented at SPE Annual Technical Conference and Exhibition, Houston, Texas, September 2004.

Abstract

The Real-Time Optimization Technical Interest Group (RTO TIG) has endeavored to clarify the value of real-time optimization projects. RTO projects involve three critical components: People, Process, and Technology. Understanding these components will help to establish a framework for determining the value of RTO efforts. In this paper, the Technology component is closely examined and categorized. Levels within each Technology category are illustrated using spider diagrams, which help decision-makers understand the current status of operations and the impact of future RTO projects. Uncertain value perception in our industry has been one of the critical issues in adopting RTO systems. Therefore, case histories are reviewed to demonstrate the impact of RTO projects. To assist RTO project promotion, we list lessons learned through case histories, suggest a justification process, and present a simple economic example.

Compaction-Induced Wellbore Failure And Fault Instability: A Hybrid Approach

Ahmed S. Abou-Sayed; Fan H. Meng; Gary Wang
Gulf Rocks 2004, the 6th North America Rock Mechanics Symposium (NARMS), Houston, Texas, June 2004.
https://onepetro.org/ARMANARMS/proceedings-abstract/ARMA04/All-ARMA04/ARMA-04-594/117703

Abstract

This paper outlines a solution approach for evaluating the stability of casing and faults due to reservoir compaction. Firstly, a geomechanics model is presented for the evaluation of casing failure due to reservoir compaction. Secondly, a threedimensional finite element analysis is coupled with the developed geomechanics compaction model for the detailed casing failure analysis. Deformations and stresses are determined on a cylindrical surface surrounding the length of the newly drilled or completed wellbore in the regions of interest. This cylindrical surface is sufficiently remote from the wellbore so that the wellbore has no or little influence on the stresses and displacements due to the reservoir compaction on this surface. The calculated displacements on the cylindrical surface are then used as boundary conditions for a focused near-wellbore stress and strain analysis using finite element technology. This hybrid analysis affords evaluating the near wellbore details that are often glossed over with a fastly compacted solution not requiring multimillion FEA cells. Yet, it preserves the fine details around the wellbore and allows for incorporating fault loading and macro influences of geologic structures and reservoir extent. It preserves the material balance and does not alter the pressure volume relationship in the reservoir void space. Interface elements can account for the slippage between the casing and the cement and between the formation rock and the cement. Field cases are presented for both the geomechanics model and hybrid finite element model.