The primary fluids encountered are brines and hydrocarbon oils and gases. Drilling, completion, and fracturing fluids can also be present, and their effects are typically studied to prevent formation damage. This page will concentrate on the role of water and, in particular, how water saturation can influence rock strengths measured in the laboratory or derived from well logs. Pore fluid pressures will reduce the effective stress supported by the rock mineral frame. This effect has been well known since the publication of Terzaghi and Peck and has been documented by numerous investigators.
Ibrahim Mohamed, Mohamed (Colorado School of Mines) | Salah, Mohamed (Khalda Petroleum) | Coskuner, Yakup (Colorado School of Mines) | Ibrahim, Mazher (Apache Corp.) | Pieprzica, Chester (Apache Corp.) | Ozkan, Erdal (Colorado School of Mines)
A fracability model integrating the rock elastic properties, fracture toughness and confining pressure is presented in this paper. Tensile and compressive strength tests are conducted to define the rock-strength. Geomechanical rock properties derived from analysis of full-wave sonic logs and core samples are combined to develop models to verify the brittleness and fracability indices. An improved understanding of the brittleness and fracability indices and reservoir mechanical properties is offered and valuable insight into the optimization of completion and hydraulic fracturing design is provided. The process of screening hydraulic fracturing candidates, selecting desirable hydraulic fracturing intervals, and identifying sweet spots within each prospect reservoir are demonstrated.
In recent years, the exploration and production of oil and gas from Bakken formation in Williston Basin have proceeded quickly due to the application of multi-stage fracturing technology in horizontal wells. Knowledge of the rock elastic moduli is important for the horizontal drilling and hydraulic fracturing. Although static moduli obtained by tri-axial compression test are accurate, the procedures are cost expensive and time consuming. Therefore, developing correlation to predict static moduli from dynamic moduli, which is calculated from sonic wave velocities, is meaningful in cutting cost and it makes the unconventional oil and gas exploration and production more efficient.
Literature review indicates such a correlation is not available for Bakken formation. This may be attributed to the extremely low success rate in Bakken core sample preparation and not enough published data to develop correlation to relate dynamic moduli to static moduli. This study measures and compares the moduli obtained from sonic wave velocity tests with deformation tests (tri-axial compression tests) for the samples taken from Bakken formation of Williston Basin, North Dakota, USA. The results show that the dynamic moduli of Bakken samples are considerably different from the static moduli measured by tri-axial compression tests. Correlations are developed based on the static and dynamic moduli of 117 Bakken core samples. The cores used in this study were taken from the core areas of Bakken formation in Williston Basin. Therefore, they are representatives of the Bakken reservoir rock. These correlations can be used to evaluate the uncertainty of Bakken formation elastic moduli estimated from the seismic and/or well log data and adjust to static moduli at a lower cost comparing with conducting static tests. The correlations are crucial to understand the rock geomechanical properties and forecast reservoir performance when no core sample is available for direct measurement of static moduli.
Ma, Xinxing (Oil & Gas Technology Institute of Changqing Oilfield Company, PetroChina) | Kao, Jiawei (State Key Laboratory Petroleum Resources and Prospecting, China University of Petroleum) | Zhou, Zhou (State Key Laboratory Petroleum Resources and Prospecting, China University of Petroleum)
Rock brittleness is a key factor to influence the fracture behavior in the formation. Therefore, it is important to evaluate the brittleness when doing the hydraulic fracturing. Previous studies provided various methods for rock brittleness evaluation. Few evaluations, however, could be applied for the naturally fractured carbonate formation because those methods did not integrate the influence from lithology, natural fractures and vugs. Hence, this paper indicated an integration evaluation method to investigate the brittleness in the naturally fractured carbonate formation.
The rock in this study was from the formation in the Ordos Basin. The brittleness evaluation method asked the experiment studies that included triaxial compression test, continuous strength test, Kaiser test and X-Ray Diffraction analyze. Based on the results, the influence of substrate properties and fractures-vugs in fractured carbonate rock are analyzed. Then a method to evaluate the brittleness of fractured carbonate rock is raised in which the stress-strain curves of rock mechanics tests, geologic microcharacter and the characteristics of fractures are considered. The method can give a better application in Ordos Basin.
The results show that the failure mode of fractured carbonate rock under the effect of confining pressure is mainly the shear failure. The facture will have an obvious effect on the strength of rock. The brittleness of fractured carbonate rock appears as the ability for resisting inelastic deformation before rupture and losing rate of bearing capacity after rupture, besides the minerals of rock and the development of fracture will influence the brittleness. With the increasing of confining pressure, the fractures tend to be closed which leads to the increasing of brittleness. However, the carbonate in high confining pressure is characterized by plasticity, the brittleness would reduce. The brittleness was used to design hydraulic fracturing work in the naturally fractured carbonate formation of Ordos Basin.
Hydraulic fracturing is necessary to guarantee a successful development in the naturally fractured carbonate formation. Therefore, the brittleness evaluation method is worth to study when designing the hydraulic fracturing.
Kumar, Rajeev (Schlumberger) | Zacharia, Joseph (Schlumberger) | Guo Yu, Dai (Schlumberger) | Singh, Amit Kumar (Schlumberger) | Talreja, Rahul (Schlumberger) | Bandyopadhyay, Atanu (Schlumberger) | Subbiah, Surej Kumar (Schlumberger)
The unconventional reservoirs have emerged as major hydrocarbon prospects and optimum yield from these reservoirs is dependent on two key aspects, viz. well design and hydrofracturing wherein rock mechanics inputs play key role. The Sonic Measurements at borehole condition are used to compute the rock mechanical properties like Stress profile, Young's Modulus and Poisson's Ratio. Often, these are influenced by the anisotropy of layers and variations in well deviation for same formations. In one of the fields under review, the sonic compressional slowness varied from 8us/ft. to 20us/ft. at the target depth in shale layer in different wells drilled with varying deviation through same formations. This affected the values of stress profile, Young's Modulus and Poisson's Ratio resulting in inaccurate hydro-fracture design. At higher well deviation, breakouts were frequently observed and could not be explained on the basis of compressional slowness as it suggested faster and more competent formation. Current paper showcases case studies where hole condition improved in new wells with better hydro fracturing jobs considering effect of anisotropy in Geomechanics workflow. Sonic logs in deviated wells across shale layer were verticalized using estimated Thomson parameters considering different well path through same layer and core test results. Vertical and horizontal Young's Modulus and Poisson's Ratio were estimated for shale layers with better accuracy. The horizontal tectonic strain was constrained using radial profiles of the three shear moduli obtained from the Stoneley and cross-dipole sonic logs at depth intervals where stress induced anisotropy can be observed in permeable sandstone layer. A rock mechanics model was prepared by history matching borehole failures, drilling events and hydro-frac results in vertical and horizontal wells using updated rock properties. Geomechanical model with corrected sonic data helped to explain the breakouts in shale layer at 60deg-85deg well deviation where the original sonic basic data suggested faster and more competent formation with slight variation in stress profile among shale-sand layer. Considering shear failure, the mud weight to maintain good hole conditions at 80deg should be 0.6ppg-0.8ppg higher than that being used in offset vertical wells. Estimated closure pressure and breakdown pressure showed good match with frac results in deviated wells using new workflow. There was difference of .03psi/ft-0.07psi/ft. in shale layers using this new workflow which helped to explain frac height and containment during pressure history match. This paper elucidates the methodology that provides a reliable and accurate rock mechanics characterization to be used for well engineering applications. The study facilitates in safely and successfully drilling wells with lesser drilling issues and optimized frac stages.
Tariq, Zeeshan (King Fahd University of Petroleum and Minerals) | Mahmoud, Mohamed (King Fahd University of Petroleum and Minerals) | Abdulraheem, Abdulazeez (King Fahd University of Petroleum and Minerals) | Al-Nakhli, Ayman (Saudi Aramco) | Bataweel, Mohammed (Saudi Aramco)
The enormous resources of hydrocarbons hold by unconventional reservoirs across the world along with the growing oil demand make their contributions to be most imperative to the world economy. However, one of the major challenges faced by oil companies to produce from the unconventional reservoirs is to ensure economical production of oil. Unconventional reservoirs need extensive fracturing treatments to produce commercially viable hydrocarbons. One way to produce from these reservoirs is by drilling horizontal well and conduct multistage fracturing to increase stimulated reservoir volume (SRV), but this method of increasing SRV is involved with higher equipment, material, and operating costs.
To overcome operational and technical challenges involved in horizontal wells multistage fracturing, the alternative way to increase SRV is by creating multiple radial fractures by performing pulse fracturing. Pulse fracturing is a relatively new technique, can serve as an alternative to conventional hydraulic fracturing in many cases such as to stimulate naturally fractured reservoirs to connect with pre-existing fractures, to stimulate heavy oil with cold heavy oil production technique, to remove condensate banking nearby wellbore region, and when to avoid formation damage near the vicinity of the wellbore originated due to perforation. Pulse fracturing is not involved with injecting pressurized fluids into the reservoir, so it is also a relatively cheaper technique.
The purpose of this paper is to present a general overview of the pulse fracturing treatment. This paper will give general idea of the different techniques and mechanisms involved in the application of pulse fracturing technique. The focus of this review will be on the comparison of different fracturing techniques implemented normally in the industry. This study also covers the models developed and applied to the simulation of complex fractures originated due to pulse fracturing.
Khan, Khaqan (Saudi Aramco) | Almarri, Misfer (Saudi Aramco) | Al-Qahtani, Adel (Saudi Aramco) | Syed, Shujath Ali (Baker Hughes, a GE Company) | Negara, Ardiansyah (Baker Hughes, a GE Company) | Jin, Guodong (Baker Hughes, a GE Company)
Rock mechanical properties are required as an input in many petroleum engineering applications, such as borehole stability analysis, hydraulic fracturing design, and sand production prediction. Their determination is commonly from various laboratory testing performed on subsurface rock samples. Due to the scarcity of reservoir samples and test cost, rock mechanical data are always very limited. Therefore, empirical correlations are very often used to estimate the mechanical properties from downhole logging measurements. Alternatively, the data-driven analytics techniques have been developed for predicting rock properties from other formation properties that can be determined directly from logs.
This paper presents a study of developing correlation equations and data-driven models that are used to predict the unconfined compressive strength (UCS) from logging data. Various rock mechanical tests including UCS, single- and multi-stage triaxial tests are performed on sandstone samples from three wells in one region. UCS values are obtained either from the UCS testing directly or from the Mohr-Coulomb failure analysis indirectly. Rock properties, such as mineralogy, porosity, grain and bulk density, ultrasonic wave velocities, are measured for each tested sample, which are used to build the correlations and data-driven analytical models for predicting UCS. Results shows that the empirical correlations are not universal and often cannot be used without some modifications, while the data-driven model is more generalized in application. In addition, data quality is very crucial for building correlations or predictive models.
Mature oil and gas wells will underperform due to different damage mechanisms and/or low permeability, and the unconventional oil and gas wells could not produce at economical rates unless stimulated successfully. The key is to understand and identify the damage mechanism and sources of low productivity in both conventional and unconventional reservoirs, and then to design economical and successful stimulation treatments. In this course, participants will first learn the fundamental science related to geosciences, rock mechanics, and fluid mechanics, and then gain know-how knowledge on the principles of well stimulations followed by practical skills related to design and evaluation of stimulation treatments. At the end of this course, participants will gain the ability and confidence in solving real-world problems by integrating physics, geology, rock mechanics, formation evaluation, production and reservoir engineering. Examples, case studies, and leading software demonstration/practices will further enhance participants’ knowledge and skills acquired in this course.
This course presents the fundamentals of fracturing pressure analysis. This includes design parameters that can be determined, uses and limitations of such analysis for on-site design, and field examples. Sessions include real world examples from a variety of environments, from "tight" gas to high permeability, offshore, and "frac-pack" treatments. Topics covered include in-situ stresses, fracture geometry, closure pressure determination, bottom hole treating pressure interpretation, pressure decline analysis, fluid efficiency, Fluid loss coefficient, pressure vs. fracture height growth—stress profile, proppant/fluid scheduling from pressure decline data. Engineers those involved in design and evaluation of hydraulic fracturing jobs.