A large number of California's Central Valley, coastal, and offshore oilfields produce from the Miocene Monterey shale formation. Example oilfields with productive Monterey intervals include offshore and inland fields such as Hondo, Point Arguello, Lost Hills, Elk Hills, Belridge, and North Shafter/Rose. Example productive formation members that produce from the Monterey shale would include the Antelope Shale, McLure shale, McDonald Shale, and the Reef Ridge. The Monterey shale is both a reservoir and hydrocarbon source rock. There are large variations in reservoir properties and general interval behavior due to differences in the lithology, diagenetic state, mechanical properties, and the stress state. Generally the Monterey shale has a low matrix permeability of 0.01–1.0 md, but effective permeability can be higher due to natural fracture conductivity. Porosity spans a huge range – from as low at 10% to as high as 70%. One illustrative example is in Southeast Lost Hills, where vertical wells are drilled and hydraulically fractured to target thick Opal-CT and the deeper cherty/quartz-phase Monterey pay intervals. The design and placement of multiple fracture treatment stages with appropriate conductivity and half-length is a requirement for a successful field development. In contrast, offshore Monterey developments have usually targeted intersecting the conductive natural fracture network in quartz-phase intervals. For this flavor of the Monterey, an acid job can effectively stimulate production by removing the drilling/completion damage and dissolving the calcite filling in natural fractures. A significant thickness of Monterey siliceous shale is present across a large area in California. The oil industry has learned much from unconventional shale reservoir development elsewhere in the country. Is it possible that Monterey siliceous shale will be transformed into the California version of an ‘unconventional’ shale resource? The key will be to identify the Monterey intervals with the most potential, and apply the proper development drilling and completion strategy, with changes made to accommodate the low-mobility multiphase liquid flow. General Monterey development challenges include: a) defining the structure, b) identifying the target pay intervals within the thick depositional intervals, c) understanding the lithology and the reservoir / rock properties of the different pay zones, d) understanding and designing the optimum stimulation treatment(s) and e) determining the optimum well development pattern and completion strategy to incorporate all the above requirements. Is there a "silver bullet" strategy that will enable success in the pervasive Monterey siliceous shale? The purpose of this paper is to stimulate industry thought and discussion.
Evaluating a cement system for zonal isolation and its ability to support the casing have always, among other parameters, used compressive strength as an important set cement slurry requirement. Recent studies have concluded that compressive strength is not the most important characteristic of the cement to provide zonal isolation and casing support. Elasticity and tensile strength of the cement system is more important for casing support and zonal isolation. In Kern county California, cyclic steam wells are a common completion method for heavy oil recovery. These wells may have temperature variations between production and steam injection cycles of up to 300°F. These extreme wellbore temperature swings create a large amount of induced stress on the cement sheath. Currently, the industry approach is to develop the highest compressive strength cement system possible to maximize cement bonding. However, advanced modeling now available indicates that such systems might not be optimized for such applications. A lightweight cement system has been developed and laboratory tested utilizing fit-for-purpose additives to give the set cement enhanced elastic and tensile properties. This paper will discuss the design and development, along with the potential application of an enhanced set-cement mechanical property system. The results of a Cement Stress Model, and the impact on cementing cyclic steam wells will also be presented. Introduction The main purpose of a cement system is to provide zonal isolation for the life of the well. Portland cement is commonly used with a mixture of additives to produce cement slurry designed to meet well requirements. Such requirements would include well depth, bottom hole static and circulating temperatures, fracture and/or leakoff pressures, pore pressure, and any other special well or operational requirements. It is typically these conditions that dictate the slurry density, fluid loss, free water, and thickening time requirements in addition to set slurry compressive strength requirements. The compressive strength requirements of the set cement is usually a value chosen by the operator, the service company, or the local oil and gas regulatory agency. It is a given that at a minimum, the compressive strength should at least be high enough to support the pipe weight at a given cement slurry height. However, it is our opinion that there is no direct relationship between the ability for a cement to bond to the pipe and the formation and the compressive strength of the set cement. A compressive strength value is chosen as important parameter of the slurry design. The strength, however, should be high enough to support the pipe as mentioned above and the slurry should not lose such strength due to retrogression. In high temperature wells, or wells that will be exposed to temperatures above 230°F, silica flour and/or silica sand is added to prevent cement strength retrogression. The above slurry requirements are designed to meet well conditions while the drilling rig is on the hole. However, it is imperative that the stress conditions on the cement sheath also be considered and quantified for the life of the well while under production and/or injection cycles. These stress conditions are mainly created due to the heating and cooling of the wellbore and the pressure cycles of injection and production. It is therefore the ability a set-cement system to provide good zonal isolation is not only dependent on the condition of the well while the cement is being placed, but also during the well production/ injection. Even though this paper is focused on cement slurry design considerations, it is important to note that there are factors that will effect the quality of even the most perfectly designed cement slurry. These factors would include poor pre-job practices, poor spacer design, poor dry blending procedures, mixing problems, and post job problems.
In many of California's shallow, low permeability formations hydraulic fracturing is necessary to enhance production rates. The low closure-stresses of some of these formations require effective proppant flowback control. Such control is not only necessary to minimize problems associated with sand production but also to preserve the integrity of the propped fracture and maintain propped fracture conductivity. Various additives and techniques have been successfully applied to control proppant flow back. These include resins, resin-coated sands, fibers, plastics, and deformable proppants. Another well production problem associated with proppant pack conductivity loss, rarely addressed by the industry, is the migration of fines from the rock matrix into the proppant pack. Fines migration reduces the permeability of the proppant pack, causing a loss of conductivity. This loss of conductivity produces a choke effect and increases the rate of a well's production decline. In California's C/D sands, in the Lost Hills field, produced fines range in size from 0.06 mm up to 0.1 mm. These fines are larger than silts and clays but smaller than fine sands. Fines production, like those generated from the C/D sands, is a natural process caused by the movement of hydrocarbons and formation water into the wellbore. An organosilane (referred to herein as "Organosilane") has been successfully applied in acidizing treatments to stabilize fines due to acid dissolution. A process was created in which Organosilane was utilized to also minimize fines migration with fractured wells. Organosilane was added at low concentrations into the pre-pad brine water injection prior to the main fracturing treatment. Based on this experience, well production results indicate the concept of adding organosilane, in conjunction with fracturing, maintained production and minimized well-pulling frequency due to fines intrusion. This paper reviews concepts and examples of treatment applications and their results. Introduction Wells are hydraulically fractured to enhance the flow potential and to accelerate reserve production. A propped hydraulic fracture is a conductive flow path super-imposed into the rock formation. This highly conductive path acts as a conduit of reservoir fluid flow from the rock matrix into the wellbore, which is equivalent to an enlarged wellbore. For an infinite conductivity fracture, the new effective wellbore radius is one half the effective fracture half-length. Fracture conductivity is defined as the product of fracture width and the propped fracture permeability. Fracture width is determined by a number of variables such as rock mechanical properties, proppant concentration, proppant amount per fracture area, and proppant size. Fracture permeability is a dynamic variable related to flow of reservoir fluids through the proppant pack. In either case, fracture conductivity is optimized with the reservoir's ability to deliver fluids to the fracture along a certain propped fracture length. This balance is known as as the dimensionless fracture conductivity.
Technical and product enhancements in well cementing have allowed the introduction of non-foamed cement systems mixed at densities as low as 7.5 ppg that attain ultimate compressive strengths as high as 1,000 psi. These non-foamed ultra low-density slurries are simplifying blending and mixing operations. In California oilfields and oilfields throughout the world, drilling wells into weak and low-pressure formations, has always been an industry challenge. Successful cement placement in these types of well environments is also a challenge. Lost circulation and partial fluid returns is major culprit to poor cement bonding in addition to higher well costs. Operators in many California fields are forced to drill wells "blind", for example, drilling without mud returns. Drilling "blind" does not preclude the challenging need to properly cement these wells to obtain zonal isolation. The process of foaming cement systems to reduce slurry densities has been applied successfully over the years in wells throughout the world. Though foamed-cementing has been successful, the design and execution add components of operational and technical challenges. These would include accurate metering of nitrogen, uncertainty of actual cement density within the wellbore, additional equipment and personnel requirements, and special drilling rig components. The need for simplicity has allowed the introduction of non-foamed ultra low-density cement systems. In California, successful cement jobs were pumped using non-foamed systems at densities as low as 10.0 ppg. In Colorado, 8.9 ppg slurry was successfully pumped that had ultimate compressive strength of over 1,400 psi. In both cases, these systems provided simpler and an economic alternative choice. These ultra low-density slurries can be designed to meet wide range well temperatures and depth applications. The paper will discuss these systems along with field applications and case histories from California and other parts of the world. Introduction Foamed cement has been successfully applied as a well cementing technique since the late 1970's. The solution of foaming cement slurry to reduce its density has solved well-cementing challenges throughout the world. The need for lower density slurry is necessitated by the need to cement across weak zones, low reservoir pressure formations, naturally fractured rocks, and highly permeable sands. The foam cementing process requires adding nitrogen (or air in some circumstances) to cement slurry to generate foam. Typically the cement slurry density is kept constant and pumped at a constant rate and nitrogen rates are increased during the job. Increasing the nitrogen rates during the job is done to compensate for the compression of the nitrogen bubble. Nitrogen gas is mixed with cement slurry at pre-designed ratios and the foam is stabilized by the addition of surfactants. Although adding nitrogen to reduce cement density has been successful well cementing process; it requires additional pumping equipment, an intricate slurry design and a complicated pumping schedule. Another method of reduccing cement density is to incorporate low density materials, such as low-density spheres, into the system. Even though low-density spheres have been available to the industry for some time, their use has not been widely applied. Recent applications, and current understanding of the pozzolanic (also known as ceramic spheres) and the borosilicate type spheres (also known as glass spheres), makes it possible to reduce the density of cement slurry; maintain desired properties, yet simplify design and field execution. Applications of the low-density sphere cement slurries have also proven to be very competitive to foamed systems. Low-density, high compressive strength cement slurry produces high performance system, allowing for the design application of a one-slurry cement job instead of multiple systems.
Hydraulic fracturing is a stimulation technique successfully applied in formations throughout the world to increase production rates and enhance hydrocarbon recovery. The process involves creating a crack by pumping fluids at pressures above formation fracturing pressures, and then filling the crack with proppant to create a high conductivity connection to a large formation area. Hydraulic fracturing stimulates production by overcoming restrictions imposed by formation permeability, drilling and completion damage, production-induced damage and, an incomplete reservoir connection across laminated intervals. The process has been applied to a large scale in many Central and Southern California fields to enable economic development and reasonable hydrocarbon recovery. Example formations include the Belridge Diatomite, Stevens Sands, Etchegoin, Antelope shale, McLure shale, McDonald shale, Point of Rocks sands, Kreyenhagen shale, Ranger sands, the UP Ford shale, and the Monterey shale. Despite the routine application of fracturing in many fields, there has been very little fracturing experience in the gas-producing formations of Northern California. Example formations such as the Martinez, Forbes, Winters, and the K-1 are generally laminated sand intervals with low to moderate permeability (less than 1 mD and up to 10 mD), that are easily damaged by completion and production operations. Despite the hydraulic fracturing potential for stimulating production rates, improving gas recovery, and increasing reserves by extending the economic development area, it has been only sparingly employed. General formation properties are reviewed -what are the implications for hydraulic fracture potential, treatment design and placement challenges? Several treatments are reviewed to provide examples of fracture treatment behavior and response. Based on the initial experience and formation properties, it is believed that hydraulic fracturing has a significant potential in many Northern California gas reservoirs.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.