Detailed model of prospects includes ground water, environmental inputs
Anchor Holm, P.E.
Anchor E. Holm & Associates
- The detailed stratigraphy of shallow sands in deepwater includes all aquitards and aquifers and their characteristics of porosity, permeability/transmissivity, aquifer volumes, fluid pressures, ground water quality, and formation strength. [5,016 bytes]
At present, well design/build teams used by industry do not appear to effectively utilize ground water and environmental engineers to work along with the geological, geophysical, drilling, reservoir, geotechnical, production, and operations staff.
With control of these ground water pressures, quality and flow potentials could be maximized at deepwater oil and gas exploration and development locations with the inclusion of their expertise. These well losses represent significant financial, environmental, and schedule losses and liabilities to the working interest owners.
The well design/build team goal is to anticipate and control the natural occurring conditions in the subsurface in a manner that ensures that wells can be drilled cost effectively to the productive zones.
Additionally, the wells must be installed with a minimum of impact upon the environment and be constructed to protect the environment until plugged. The wells must also be designed for effective abandonment upon depletion of the oil and gas mineral resources.
This article will focus on the planning and well installation phases of the site, with an eye on future needs and cost savings that can be built-in during well installation. Control and reduction of overpressure in shallow sands is a key objective that can significantly reduce installation costs and other liabilities that the operators have been experiencing.
Careful evaluation of the seafloor and the first 1,000 meters below the mudline is needed to define the natural conditions in the site area including overpressured shallow water sands.
Historical perspectiveThe petroleum industry has determined that shallow gas sands as well as overpressured shallow water sands may be present in the first 1000 meters below the mudline in deepwater areas. Shallow gas flows have caused serious concern to the offshore drilling and production personnel.
The gas flows have caused fire, safety, floatation, and foundation hazards to offshore drilling and production operations. Frequently, these gas flows bridge off, stopping uncontrolled releases at the seafloor after some period of time, thereby minimizing the duration of effects upon the site, equipment, and environment.
Recently, shallow water flows have become more of a concern. The water flows generally have not created extreme hazards to platform personnel or surface equipment. However, the loss of wells and wellheads at the deepwater seafloor have become costly in terms of potential environmental liabilities, lost production, and well replacement costs.
Experience has shown that these water flows do not bridge off in the same fashion as do the gas flows. They can flow more persistently at the seafloor and may invade shallower normally pressured sands, creating a secondary drilling or foundation hazard.
Shallow sand water flows have been reported in the Gulf of Mexico, offshore of West Africa, west of the Shetland Islands, Borneo, and Southeast Asia. They appear to occur in deepwater sites where the depth of seawater is more than 500 meters, just beyond the continental shelf edge, or along submarine canyons incised into the shelf.
Many wells have been lost during installation, and a lesser number of producing wells have been lost during the early portion of their planned productive life. A conservative estimate of $2 million dollars per lost well to abandon and replace has been reported.
To prevent these lost well costs and minimize the sea floor ecological effects, the approach to planning and design as well as installation can be expanded slightly with significant savings. The site conceptual model definition and expansion follows.
Enhanced site modelDevelopment of the site conceptual model for each drilling site is a dynamic process. It begins during the initial exploration efforts, follows into development, extends throughout the productive life of the site and post abandonment.
This model includes at a minimum the geology, hydrology, topography, ocean floor ecology, ocean currents, shipping lanes, fisheries, historical sites, exploratory drilling, platform installation, development drilling, oil and gas production, production storage and transportation, liquid and gaseous emission control, waste disposal, sediment consolidation, sea floor subsidence, sea and geothermal gradients, salt formation influences, as well as present and future regulatory requirements.
Industry is considering all of these in each site conceptual model to various degrees, plus economics and other issues unique to the site. This clearly demonstrates the need for the use of multidiscipline design/build and operating teams by the offshore industry.
Focusing upon the shallow sand flows, the model must include the detailed shallow stratigraphy of the unconsolidated normally pressured as well as the overpressured beds.
This detailed stratigraphy must include all aquitards and aquifers, and their characteristics of porosity, permeability/transmissivity, aquifer volumes, fluid pressures, ground water quality, and formation strength. It is very doubtful that any aquicludes (totally impermeable beds) will be present.
Prior to drilling, a limited amount of information on some of these conditions can be determined, the majority are initially estimated. Most must be measured, sampled and characterized during or just prior to drilling into each aquifer. This allows the control measures to be applied efficiently in a timely manner.
Depending upon the seafloor water quality, temperature, topography and ecology, release of slightly warmer ground waters (from the overpressured shallow sands) into the sea floor may be acceptable. However, mixing of the different waters may cause undesirable reactions to occur. Salinity and density differences may affect dispersion, mixing and local ecology, as well as increase the corrosion potential for any seafloor equipment.
Evaluation of the risks financially and environmentally must be calculated carefully and reviewed for each option to control the overpressured sands. The initial option would be to work with nature and release the overpressure waters into the wellbore and sea floor.
Another option is to prevent any of the shallow sand fluids from entering the wellbore, This option must prevent the migration of these fluids up the annulus of the well and from becoming an uncontrolled flow upwards into the sea floor.
Design requirementsIf the ground water can be discharged onto the seafloor or otherwise handled efficiently, a pressure reduction system for the overpressured aquifer in the vicinity of the site can be designed. Input to the design of the system(s) will include a ground water flow model that can be updated with new data efficiently. Management of the aquifer pressures is essential if the installation of new wells is to be successful.
Pressure relief wells or annuli of producing wells will be designed so as to control the flow from the overpressured sands with a minimum head requirement at the seafloor datum. Location of theses wells will be key to effectively reducing the head in the aquifer.
The length of time of the ground water flow will be determined based upon the further need for keeping the overpressure reduced as the oil wells are produced. Also, the water discharged and its quality can be remotely monitored and metered. This allows the operator to control the effects upon the seafloor ecology.
Whenever a well experiences a shallow water flow discharging outside the casing and/or conductor pipe, the ground water is an uncontrolled release or seafloor eruption. This release will not likely completely bridge off since water fluidizes sediment significantly different from gas flows.
There is little opportunity for the sediment to collapse into a pocket filled with water, as compared to one previously filled with gas. As a result, the ground water will likely continue to be released at the sea floor in the vicinity of the original wellbore.
Over time, this will reduce the amount of overpressure in the aquifer locally. This may allow subsequent oil wells to be installed nearby without the shallow water flow eruption. However, there exists no designed opportunity for the operator to discontinue the release with time.
A data management plan must be included in the design. This plan will manage four (4) phases of data collection: exploratory, development, operations and post-abandonment.
- The exploratory phase will provide many of the input parameters for the development design of the site.
- During the development phase, the new data will trigger contingency plans and refine the design in a real time mode.
- During the operations phase, the data will provide monitoring of the sea floor ecology and discharges. This will allow the operator to vary or cease ground water releases as needed with little cost involved.
- Post-abandonment monitoring will allow the operator to select the most prudent monitoring locations and obtain early closure of the site.
Installation requirementsThe site conceptual model is a basic input to the overall installation design and construction planning. In areas containing shallow water flows, the long term support of wells and seafloor equipment is a prime consideration, along with any potential effects of those waters upon the sea floor ecology.
This will require a focus on the interval from the mudline down into the overpressured aquifer(s) during the drilling and casing setting of oil and gas wells. From the mudline down to the top of the overpressured sand, the openhole must have a minimum of washouts, i.e. low rugosity.
Keeping this shallow portion of the openhole in gauge requires skill and attention to the drilling parameters. However, the payoff is an effectively cemented casing that can withstand the overpressured aquifer's potential to flow upwards via a leak in the annulus. This is needed on all wells drilled, including any installed as part of the pressure relief system.
The completion of relief wells must be hydrodynamically efficient (excellent flow efficiency for high volume; low pressure drop releases). The overpressure sands have generally demonstrated excellent transmissivity. As a result, a pumping system in the relief wells should not be required if the wells are designed and installed properly. They must also be capable of producing sand-free after proper development.
During operating conditions, the pressure relief system must maintain a net positive flowing pressure (relative to the seafloor pressure) on the overpressured aquifer. Maintaining this safe pressure head on the sands of concern will prevent any seawater from entering the system and require all mixing of waters to occur at the sea floor. Thus, any geochemical instability caused by the mixing will not create costly internal maintenance or other operating problems.
By having multiple relief discharge points, any adverse effects on the seafloor ecology can be mitigated effectively. Location of such points will be a function of the sea floor topography and sea currents. Use of intermittent flow from the various discharge points may greatly reduce any seafloor effects, while maintaining the lowered overpressure in the sands of concern.
ConclusionsIndustry has experienced considerable well construction costs in the order of $2,000,000 per well lost due to overpressured sand, shallow water flows in the deep-water offshore provinces around the world. Additionally, the lost production and time delays in bringing wells on production have incurred costs affecting the economic performance of the offshore prospect.
The fluctuation of oil prices, and to a lesser degree, gas prices has significant impact upon the field economics and desired timing of peak production performance. Delays caused during installation of wells can cause cost overruns and loss of sales during peak pricing. Therefore, control of these overpressured sands is critical to the economics both in the short term and the long term.
Ground water flow modeling of the overpressured sands must be done to evaluate and design the most effective pressure relief system for each platform and producing site. The modeling will be required to design the completions of any relief wells to operate with a maximum of efficiency over their operating life. Locating of the relief wells will require model iterations. Initial bottomhole pressure and subsequent flow testing of the wells will be required during installation of the relief system and oil wells.
The overpressured shallow sands found in deepwater offshore are frequently associated with shallow salt intrusions or beds. Whenever this occurs, the quality of their ground water is more saline than the seawater occurring at the ocean floor.
Modeling of the geochemical mixing of the two waters should be done to evaluate their effects upon the wells and related equipment, as well as the sea floor and its ecology. Inclusion of ground water engineers and environmental engineers on the design/build teams would be very beneficial to the oil industry on offshore and large onshore projects.
The ground water engineers and scientists are trained and experienced with flows in confined as well as unconfined aquifers. Few petroleum engineers have such experience. Environmental engineers and scientists are trained in fluid mixing, ecology, sampling using defensible techniques, as well as regulatory requirements for present and future compliance.
AcknowledgementThe author wishes to thank A. Joseph Reed, Senior Hydrology Expert, Arcadis - Geraghty & Miller, for technical review of this paper, and M.A. Holm for editing and critique.
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Mr. Holm is a registered professional engineer and has over 30 years of engineering experience, including petroleum, ground water, and environmental. For the past nine years, he was with Geraghty & Miller in Midland and Albuquerque. Currently, he has an engineering consulting practice in Houston. He holds a BS in Geological Engineering from the University of Arizona and has completed 36 hours toward an MS in Environmental Engineering at U.T.- El Paso.
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