Novel multizone stimulation technologies have enabled the development of tight resources that previously could not be developed economically and have enabled optimization of production resources distributed over thick gross intervals. These technologies are expected to become more broadly used for numerous applications worldwide because of their ability to rapidly place numerous stimulation treatments tailored to the specific needs of each zone, pump each treatment efficiently and effectively, and make more efficient use of equipment, people, and surface-site development.
Introduction
Unique stimulation challenges arise when hydrocarbon resources comprise multiple vertically distributed discrete reservoir intervals contained in long productive intervals or when targeting tight reservoirs with horizontal wells having extended lateral sections in reservoirs with significant heterogeneity. The overall challenge is characterized by the need to manage a balance between the number of stimulations performed, stimulation-treatment quality, cost, and the overall reliability of treatment placement and execution. Currently, managing this balance can result in operators intentionally bypassing less-attractive hydrocarbon intervals, incurring lower production because of poor stimulation effectiveness, labeling resources as uneconomic on the basis of the cost required to access the reserves, or having difficulty in achieving reliable and effective treatment placement. During the last decade, extensive research and field experimentation enabled the development of unique workflows, modeling capabilities, and stimulation methods for tight sandstone, shale, and carbonate formations.
Hydraulic-Fracture-Stimulation Design Methodology
The stimulation design methodology revolves around identification of the optimum completion scenario for the given reservoir conditions and local situation. Compared with conventional reservoirs, unconventional reservoirs with low matrix permeability, low porosity, and high water saturation may be more prone to damage and flow impairment.
Extensive field experience and measurements show the effective fracture volume is always less than the induced one. The rock is anisotropic and heterogeneous. In some cases, simple biwing-like fractures may be induced if the stress anisotropy is high; alternative fractures could be complex clusters with low anisotropic stress state. It is crucial to determine optimum completion scenarios, including horizontal longitudinal, horizontal transverse, or multizone vertical stimulations (Fig. 1).
A unique set of software tools and integrated design workflows are used to develop and optimize the stimulation-treatment design. The first step of the design is to select the appropriate optimum completion, target zones, and stimulation sequence by use of available analogs based on the field information, including reservoir characteristics.
Early data collection in unconventional tight reservoirs is essential for assessment, development, and stimulation optimizations. Limited or partial data availability for design is a very common occurrence, especially during the early exploration stage of the field. When data are limited, available analogs, empirical correlations, and relevant data need to be integrated and correlated to obtain necessary information and extensive sensitivity studies are conducted. It is equally common to have multiple sources for single data or superficially contradicting data, emphasizing the need for a physics-based, integrated, and optimized workflow even at the stimulation-design stage of data preparation.
With the optimum completion scenario and all available data, software is used to analyze and integrate all information, including petrophysical, geological, core-test data, correlations to prepare high-confidence rock properties, and stress information with a physics-based layering approach for optimization.
Other important considerations to maximize productivity are possibly to limit fracture growth to avoid potential water-productive zones; treatment designs to maintain fracture containment within the target zone; and fracture design to potentially cover multiple productive zones that are adjacent and close to each other. Finally, the recovery prediction is obtained on the basis of obtained fracture-design geometry in conjunction with economic analysis.
Implementation and Diagnostics
Common completion methods for stimulation of thick intervals or multiple reservoir targets include the limited-entry method and the plug-and-perforate method and can be applied in vertical, deviated, or horizontal wells (Fig. 2).
While these methods are widely preferred for operational reliability, the effectiveness of treatment placement may be reduced if there is substantial formation or reservoir heterogeneity present. A key diagnostic that has enabled improved understanding and development of improved design approaches and new stimulation approaches is a microseismic fracture-mapping tool that uses acoustic methods to infer the geometry of induced hydraulic fractures. This tool consists of an array of three triaxial geophone receivers, a telemetry sub to transmit the data to surface in real time on conventional seven-strand wireline, and an interface to a gyroscopic survey tool to provide the tool orientation when clamped.
Improving Control of Treatment Placement: Novel Selective-Stimulation Technologies
During the last decade, extensive research and development coupled to field experimentation have enabled development of several new technology solutions that provide more control of stimulation-treatment placement, reduce the surface footprint, and allow implementation at an equitable or even more-competitive cost structure when tailored for the local conditions.
These technologies include
- Just-in-time-perforating (JITP) stimulation
- Annular coiled-tubing fracturing (ACT-frac) stimulation
- Simultaneous operations
- Novel design methodologies for carbonate stimulation
JITP
JITP involves pumping high-flow-rate treatments down the casing with a perforating-gun assembly in the well to perforate one interval at a time. Diversion between treated intervals is promoted through the deployment of ball sealers. A well is stimulated using the JITP method by sequentially treating individual zones by the combination of selectively perforating single intervals and promoting the isolation of these intervals through the deployment of ball sealers. A single continuous pumping operation allows uninterrupted operations.
Fig. 3 shows the JITP process. If zones have already been stimulated, the gun assembly is run with a bridge/fracture plug and setting tool and the plug is set below the first zone to be stimulated. After setting the plug, the gun assembly is positioned at the first zone and the first gun is fired. The gun assembly is then raised to the next zone of interest, and the first treatment is pumped. At the tail end of the treatment, ball sealers are added to the flow, with at least one ball sealer for each perforation. When the ball sealers arrive downhole and seal the perforations, as indicated by a sharp rise in wellbore pressure, the next gun is fired and the second treatment is initiated without ever shutting down the pumps. This process is repeated for as many zones as there are individual guns on the deployed assembly. Once all the target zones have been stimulated, the gun assembly is retrieved. This process is repeated until the entire well has been completed.
ACT-Frac
ACT-frac was developed with the objective of capturing further enhancements to stimulation-treatment diversion. The ACT-frac approach relies on the deployment of a bottomhole assembly (BHA) that permits multiple treatments with a single BHA deployment into a well.
A well is stimulated using the ACT-frac method by sequentially treating individual zones, starting from the bottom (or toe) of a well and moving uphole (or toward the heel). Each zone is treated separately using a single pumping event without the use of bridge plugs or fracture plugs. Positive isolation is achieved using multiset downhole equipment that is moved uphole from zone to zone to facilitate multiple treatments with a single deployment of wellbore equipment into a well. Fig. 4 above illustrates a typical ACT-frac stimulation process.
The packer diversion method provides leak-tight positive isolation between the interval being stimulated and the intervals previously stimulated; this allows a single pumping event to place a tailored interval-specific treatment while ensuring that previously placed stimulations are not damaged or altered by subsequent stimulation treatments. The technique is suited for application in deviated and horizontal wellbores.
This method has been identified as providing significant performance uplift in multizone-stimulation applications. This technique may find the most beneficial utility for application in shallow wells or when smaller-volume stimulation treatments may be required and the number of fractures per well is limited.
Simultaneous Operations
Simultaneous-operations technology has enabled substantial benefits from operational efficiencies associated with multizone-stimulation operations. Specifically, the technology enables sequential pumping operations on multiple wells without suspending fracturing operations to wait for nonpumping events (e.g., wireline rig up and running and setting fracture plugs).
The major components in the simultaneous operations are a stimulation-fluid storage system, a stimulation-fluid pumping system, a manifold system to enable stimulation fluids to be directed to any of the wells on the pad, flowline manifolds, two cranes, two wireline units, perforating tools, and a coiled-tubing unit. Multiple wellheads are connected from the high-pressure pumps through a single manifold. By configuring the valves before pumping fluids and proppant, the stimulation operator can choose which well will be treated and avoid the potential hazards and time delays associated with rig up and rig down of high-pressure lines multiple times over the course of many days. Individual choke manifolds are installed on each well to accommodate flowback and recovery of fluids used to transport the proppant through the wellbore.
Novel Design Methodologies for Carbonate Stimulation
For matrix acid stimulation of carbonate reservoirs, additional unique challenges may be encountered associated with increased rock heterogeneity, development of differential reservoir pressures, and effect of the hydrostatic pressure of the fluid in the wellbore. Effective stimulation of moderate-thickness carbonate intervals has been achieved through a combination of high-rate injection, viscous diversion, and selective perforating, to minimize the effects of heterogeneity.
Viscous-fluid diverter systems have been demonstrated to greatly improve the matrix stimulation and acid distribution in moderately thick carbonate reservoirs (200–600 ft) penetrated by vertical (and deviated) wells. In general, viscous diverters are either polymer based or surfactant based. The preferred fluids for diversion in carbonate-matrix acidizing are those that develop viscosity upon reaction with the formation. A low initial viscosity allows the diverter to follow the preceding acid stage. Upon reaction with the carbonate rock, viscosity increases to help divert the subsequent acid stages to an unstimulated portion of the completion. To further improve acid distribution and ensure optimum stimulation of lower-permeability layers, selective perforating is used as an integral part of the stimulation strategy.
Autonomous Methods for Optimizing Multizone-Stimulation Operations
Given the significant performance improvements that have been enabled by JITP, ACT-frac, selective perforating and viscous-fluid diversion, and simultaneous operations, the next step change in technology for operational enhancement of selective stimulation is now being pursued with the development of autonomous delivery systems, where the focus is on the complete removal of the downhole-tool-conveyance methods—specifically, elimination of the need to use wireline, coiled tubing, or jointed pipe as part of the multizone-stimulation method.
Elimination of the wireline or tubing can offer further significant operational enhancements because the overall surface footprint can be reduced with elimination of large cranes, wireline units, and coiled-tubing units because these are no longer necessary for deployment or control of the downhole selective-stimulation tools.
An autonomous perforating tool (Fig. 5) is being developed that entails a self-destructing disposable perforating tool run in a well without a wireline. The design and construction provide for a hermetically sealed unit, with all components—e.g., perforating charges, batteries, logging devices—integrated within the device and sealed from exposure to wellbore fluids. To achieve the functionality, the perforating tool will contain essentially frangible components that will destruct into small pieces upon perforation.
This article, written by Editorial Manager Adam Wilson, contains highlights of paper IPTC 16865, “Improved Methods and Workflows for Multizone Stimulation,” by Kris Nygaard, SPE, ExxonMobil Production; Shekhar Gosavi and Pavlin Entchev, SPE, ExxonMobil Development; and Fuping Zhou, SPE, Wadood El-Rabaa, SPE, and Chris Shuchart, SPE, ExxonMobil Upstream Research, prepared for the 2013 International Petroleum Technology Conference, Beijing, 26–28 March. The paper has not been peer reviewed. Copyright 2013 International Petroleum Technology Conference. Reproduced by permission.