Medina, Leonardo Arias (Universidad de America, Bogota) | Lozano, Henry Andrey (Universidad de America, Bogota) | Mantilla, Hernan Dario (Ecopetrol) | Espinosa Mora, Carlos Alberto (Universidad de America, Bogota)
SUMMARY: After the success of the drilling campaigns in unconventional shale reservoir in the United States, Ecopetrol wanted to replicate their success in Colombia as well. During the past years a drilling campaign encountered several unique issues for shale plays in Colombia. With the advances in electrical logging, petrophysics, geomechanical and geochemical analyses allowed understanding the shale plays in a basin in Colombia.
Geomechanics has been useful in providing the required information for the design of optimal well trajectories for efficient development of unconventional reservoirs. Additionally, extensive tri-axial and total carbon organic (TOC) tests in shales have been included in the study to calibrate the mechanical properties and TOC obtained from the electrical logs. The concept of critically stressed fractures has been included in the analysis of this geomechanical model in order to know the conductivity of the natural fractures in shale plays.
The present study displays a methodology for the design of a mud weight window as well as a geochemical analysis for the sweet spot selection in the shale plays. The geomechanical model considers transversal vertical anisotropy (TIV) with the use of Stoneley wave from sonic scanner tool necessary for the determination of the anisotropic mechanical properties and in-situ principal stresses. The present study includes conclusions and recommendations for unconventional shale reservoirs in Colombia.
1 INTRODUCTIONOil companies have always wanted to drill wells in unconventional fields, but because of their complexity and limited technology in the time were an impediment.
SUMMARY: The objective of this work is to find and study the problems with the challenges that Mexico and Brazil have faced in order to obtain hydrocarbons from salt rock in the exploration, drilling and complete stages and the measures have taken for their solutions. Methodologies adopted to carry out this research include: reading papers, scientific journals, books, graduation projects, industry reports. Most of the information was collected at PUC-Rio and Petrobras, in Brazil. The research is focused on data acquisition, mud contamination, cementation, and the phenomenon known as affluence of salt. When taking seismic data from areas with high noise potential due to the presence of salt rock, it is recommended to use a WAZ technique in the first instance to avoid cost overruns. Concluding, a good characterization of the type of salt is very important, verify the composition and determine how clean or dirty saline rock is, because its behavior facing changes of temperature and pressure varies according to its chemical composition. This will allow the best measures to be taken in order to avoid potential problems such as inflow or increase in the rheological properties of the mud.
Due to the problems presented when drilling saline deposits and the belief of the absence of hydrocarbons under those structures, these formations were for a long time avoided and little studied. However, the decline in reserves and discoveries that were made in West Africa, North Sea, Mexico and Brazil, made this type of structures attractive. This is the case of Brazil, which after the great discovery of the pre-salt has developed new methods of data acquisition, new theories of formation and migration of salt, as well as innovation in techniques and equipment in order to reduce the uncertainty, risks and costs. This paper compiles a review of methods, techniques and possible solutions to problems that are faced daily by geologists and engineers in the Gulf of Mexico and Brazil.
SUMMARY: During the initial evaluation phase of an exploration project in unconventional reservoirs the intervals that will be completed should be prioritized, as a consequence, it is necessary to create a methodology to optimize the selection of intervals for stimulating. This selection is made more difficult at the beginning of the exploratory phase, especially in very thick formations (greater than 500 ft), where there may be several prospective intervals from the point of view of rock quality. Initial tests in vertical wells have a higher investment risk associated with the completion of unfavorable horizons for fracturing, i.e. failures during execution the hydraulic fracturing or great difficulty for proppant admission, therefore an inefficient fracture is created for migration of reservoir fluids into the well. As a result of interdisciplinary work, a methodology for selecting horizons with greater probability of success in hydraulic fracturing (under the Colombian tectonic environment) and for incorporating resources in a vertical well in an unconventional reservoir that has several prospective horizons was generated. Additionally, for the horizons with potential production of gas and gas condensate, a prioritization criterion was established, taking into account the effect of pore pressure decline on the matrix permeability (stress sensitivity).
The proposed methodology is based on: a) generate and interpret an anisotropic geomechanical model, b) selecting intervals that meet the quality criterion for completion in terms of elastic properties, c) generate a probabilistic solution for the distribution of the minimal horizontal stress, fracture gradient and net pressure, d) classifying selected intervals based on the likelihood of having a favorable complexity fracture, e) classifying the intervals in terms of mobility of fluids in the reservoir, f) sorting the prospective intervals in terms of value generation associated with each resource. By applying the methodology it is achieved for example discard prospective horizons with good rock quality, high probability of success of completion but low incorporation of resources, another possibility is to discard horizons with good rock quality, good fluid quality but low probability of successful completion. As a result of the exercise the operator managed to prioritize investment in 3 of the 6 prospective horizons for completion and testing.
SUMMARY: Wellbore instability during drilling cause substantial problems in different areas of the world, even in vertical wells. These problems occur when the near wellbore stress exceed the rock strength. To prevent these problems must establish a balance between stress and the strength of the rock, which must be restored or maintained during drilling through the proper formulation of drilling fluids, mud weights, trajectory of well and drilling practices.
Wells drilled in laminated shales or sequences of shale-sand, are classified as more unstable than those drilled in homogeneous and isotropic formations, this because the laminated materials may fail in two ways, first as intact rock where material strength is greater, and the second as sliding bedding planes, where resistance is lower. The way in which the failure of the rock is generated will be strongly influenced by the wellbore trajectory that has the with respect to the planes of weakness of the rock.
For the evaluation of the impact that can have the presence of the planes of weakness in the wellbore stability, a numerical modeling with a commercial software called the ABAQUS, which has a constitutive material model called "Joint Material", by which you may include a anisotropy in the rock material modeling, with which it represents the planes of weakness, in resistance and frictional properties. The implementation of this model to determine the mud weight that which the rock failure, either by planes of weakness or intact rock and have the sensitivity of what may be the volume of rock affected under certain conditions of mud weight.
This article presents comparison between collapse pressures obtained with analytical methodologies, the influence of the planes of weakness in the geometry of the fault and a field application.
SUMMARY: The mud losses events in the reservoir phase at Castilla field (Colombia – South America) during drilling campaign of 77 wells in 2015 represented an overrun close to $20 MMUS and an accumulated volume of 160.000 bbls of drilling mud lost in the hole. The main reason of these events are associated to crossing structural lineaments during drilling such as faults and set of natural fractures together with the already depleted reservoir sandstone, which results in a drop of the magnitude of the minimum horizontal stress also known as closure pressure of such natural fractures. As a result of the decreased magnitude of the minimum horizontal stress, the natural fractures are considered to be in a critical state of stress increasing the risk of having mud losses through the opened natural fractures. Ecopetrol’s geomechanics team has led the visualization and conceptualization of such mud losses events from ant-tracking models got from seismic data and plotted on the prospects wells surveys. A total of 18 wells scattered in the field were analyzed using the proposed methodology in this paper showing 78% of effectiveness of the prediction mud losses during drilling.
Based on the analysis of the drilling history of 442 wells drilled from 2005 to 2015 it was possible to point that 17.5% of the time was associated to non-productive time (NPT) and 18% of it was due to mud losses events. In Ecopetrol’s oil fields seismic attributes have never been used before to perform root cause analysis of mud losses events in reservoirs with high fracture density. Detailed seismic structural interpretations have been developed in order to improve the root cause analysis of the mud losses events in the reservoir phase using seismic attributes such as ant-tracking having in mind that these attributes are specific measurements of geometrical, cinematic, dynamic or static features that come from seismic data and are mainly used to quantify the amplitude and geomorphological features seen in seismic data.
SUMMARY: Geomechanical simulations are a powerful tool to forecast caprock deformation and failure behaviour. However, a number of drawbacks associated with simulation are often cited in dissuading their use as a major tool for caprock integrity assessment. This paper will explain that these drawbacks are not inherent in simulation itself. If vigorous efforts are exercised, there are means to overcome these drawbacks. Three approaches are presented: deterministic, probabilistic, and joint inversion. Theoretical principles as well as case histories are given to support observations: (1) forward deterministic simulations are still valid in yielding accurate results that compare well with field observations; (2) probabilistic simulation is a powerful tool to quantify the impact of uncertain material properties and their spatial variability; and (3) the data-intensive nature of a thermal project is an important asset that can be used by a joint mathematical inversion system to make inferences about evolving subsurface processes.
Whether or not it contains resources of economic value, every interval of subsurface rock formation is invaluable and their integrity must be safeguarded. If containment integrity of the caprock above a petroleum reservoir is damaged, reservoir fluid can escape into undesirable locations. This is particularly essential for heavy-oil development because it requires reservoir stimulation through injection of steam, solvent, and other chemicals to reduce the oil viscosity. For example, cumulative energy injected into a reservoir and thus stored at the subsurface during a thermal project is too significant to ignore the caprock integrity issues. If not managed properly, this energy source can harm us socially, environmentally, and economically.
Several incidences of steam release, drilling blowout, reservoir fluid escaping into shallow depths including into groundwater aquifers, or reservoir fluid escaping to the ground surface have been reported publically (Smith et al. 2004; ERCB 2010; AER 2013). More common is the significant casing deformation during thermal operation. Moreover, concerns are raised about the significant uneven surface heave that may alter facilities, roads, landscape, and surface/subsurface hydrogeological conditions.
Caprock integrity eventually becomes a geomechanical issue, even though it is first a concern of hydraulic integrity (i.e., no reservoir fluid should escape through the caprock into more shallow areas). Naturally, such hydraulic integrity is already inherently placed in-situ in the geological history because the caprock has prevented any further upward movement of hydrocarbon migration.
SUMMARY: This paper will provide an overview about the use of geomechanics in improving heavy-oil production. If high recovery factors are sought, production of heavy-oil reservoirs requires stimulation to reduce oil viscosity. Intelligent use of geomechanics can create additional porosity and permeability by managing rock dilation and fracturing behavior. As a result, new areas are created for injected stimulants to contact the heavy oil, speeding up viscosity reduction and improving oil production. Additionally, the same mechanism can break down the permeability barriers in the reservoir for the injected stimulants to travel through, ultimately improving the reservoir recovery factor.
Using field examples, this paper illustrates theoretical mechanisms and field results. It will hopefully provide a new paradigm for operators when planning their heavy-oil production, because geomechanics is both a necessity and a means to value creation.
Becoming an increasingly important component of global energy supply, development of heavy-oil resources has attracted world attention. Heavy-oil development in Canada and Venezuela, as well as in other countries such as China, the former Soviet Union, Indonesia, Oman, Russia, and the U.S., are some well-known examples, although the list is definitely not exhaustive. In the recent years, Kuwait, a traditionally conventional oil producer, has embarked on an ambitious mission to develop its heavy-oil resources. Therefore, it is timely to have the worldwide geomechanics community turn their attention toward heavy-oil development and how geomechanics can proactively help improve production rates, increase reservoir recovery, and minimize its environmental footprint (on air, land and water).
SUMMARY: This study proposes a workflow, to combine data from logs and cores to establish information of exploratory and production interest, which allows visualizing the results of the integration of geochemistry data with geomechanics in gas or petroleum prospects in shales (Shale Gas or shale oil). Correlation was performed by conventional statistical analysis. The laboratory data used for geochemistry were organic content measurements TOC, Tmax, grain density, S1, S2 and S3. The geomechanical parameters were the compressional velocity Vp, the shear velocity Vs and some static elastic moduli. The logs used were: measurements of gamma ray, sonic, density, neutron porosity and resistivity. Then parameters such as: dynamic Young's Modulus (E), dynamic Poisson Ratio (ν), organic TOC content (Passey, Tristan and regression methods) and fragility (Rickman) were calculated. Correlation was performed using statistical correlation methods (regression, correlation coefficient analysis, principal component analysis, etc.) and combination of variables according to the literature review, for example, TOC vs Young's Modulus, TOC vs Poisson ratio, TOC Vs Brittleness.
The trend observed in the behavior of the Young's Module (E) with respect to the organic content (TOC) follows an inverse relationship. For the data from the Colombian wells examined this relationship is true in most cases.
The analysis of the data does not provide a clear behavior pattern, so there is not a good understanding of the effect of the organic matter content with respect to the Poisson ratio. The same situation was found in the literature review.
Fragility is a parameter that is related in a complex way to lithology, mineral composition, amount of organic matter, effective stress, reservoir temperature, diagenesis, thermal maturity, porosity and type of fluid (Wang & Reed, 2009). Some authors discard the presence of organic matter, but it is clear that the organic matter is a constituent part of the matrix (especially in shale rocks producing gas or petroleum), the fragility of the rock is affected. It is necessary that mechanical anisotropy is also sensitive to the kerogen content. This situation was observed with data from Colombian wells.
SUMMARY: To ensure a successful drilling is essential to design a geomechanical model in order to determine a mud weight which avoids problems such as well collapse, flux of cavings and among other aspects that contribute to increase costs and non-productive times (NPT). Considering that conventional wellbore stability models don’t include the effect of circulation losses through fractured zones, this research aims to design a new algorithm that models the phenomenon of circulation and filtered loss through fractured formations. Additionally, an integrated model which includes aspects like: fracture deformation, mud rheology, and flow through fractures is implemented. This new algorithm takes into account the most important considerations in this area published in the literature The algorithm proposed in this research enables to solve mathematical models analytically and numerically in order to analyze all the possible solutions integrated in the model which includes differential equations for the fluid flow and analytical equations for the formation properties, fracture behavior and mud rheology. This new algorithm allows to determine fracture widths in real time, using information of the circulation loss or the required mud weight, if the known fracture width to maintain losses at its minimum permitted movement. These results are important to select the lost circulation material (LCM), reservoir engineering, secondary permeability log and the operation simulation can be optimized. This paper extents the knowledge of naturally fractured reservoirs especially during drilling, and modeling the phenomenon of circulation lost using a new algorithm that solves an integrated model is easy to implement.
Based on Ali Ghalambor et al (2014), the circulation loss could be the major problem during drilling increasing the non-productive time (NPT). According to Murchison, (2006), Ivan et al., (2003), this NPT cost to the petroleum industry US$ 800 million per year approximately.
Ghalambor et al (2014), also established that this circulation loss occur through the rock matrix, natural and induced fractures, and vugs. With the implementation of mathematical models to represent this phenomenon, the flux is classified into filtered through the rock generating a mud cake along the wellbore wall and flux through the fractured zones regarding the mud rheology.
Parn-Anurak and Engler (2005), studied the filtered loss through the rock matrix considering the filtered influence radio and cake thickness. This model analyze the erosion and formation rate of the cake constituted by mud filtered.
SUMMARY: This work presents a methodology for mud losses mechanism evaluation based on geomechanics of fractures. Several and catastrophic mud losses events are continuously experienced during drilling the 8½¨section in Castilla Field in Llanos basin, Colombia. Technologies like Manage Pressure Drilling (MPD), thixotropic fluids, LCM (Lost Control Materials), ECD (equivalent circulating density) management were applied to avoid/manage mud losses but the issues associated to mud losses continue being a major problem causing among others wellbore instability in K1 superior formation due to fluid static column variations. According to the events, wellbore instability becomes the new problem causing hole cleaning issues, tight hole and restrictions tripping drill pipe and 7¨ liner. In image logs were detected several natural fractures both open and partially open. Fracture´s hydraulic conductivity hypothesis was proposed. To better understand the problem an evaluation of critically stressed fracture analysis was conducted by estimation of normal and shear stresses in each fracture plane assuming pressure transmission from the wellbore to the fractures. Geomechanical parameters estimated for each interval in which fractures were identified, entered the analysis as an input. Then, the fracture´s stresses were compared to the rock´s failure envelope assuming no cohesion in the planes. As a result, was figure out a reactivation gradient, which is compared to the pressure losses estimated based on the static column height in wells that experienced mud losses. The main observation is that there exists a fracture reactivation pressure lower than the minimum horizontal stress gradient and close to reservoir´s pressure that if is overcome, mud losses take place.
Fractures are discontinuities that create escape paths for drilling fluids and thereby constitute an important mechanism of lost circulation. Most rocks contain fractures of various sizes from micro cracks at grain level to fractures extending for hundreds of feet in the reservoir. In some reservoirs, fractures provide important pathways for the reservoir fluids. Connectivity of the fracture network is its essential property. In the lost circulation context, it affects how much drilling fluid can be lost. In natural fractured reservoirs, the availability of a connected fracture system is essential for production, but is detrimental for drilling (Lavrov A, 2016).