Pipelines and Treatment Facilities Provide Options for Responding to Mid-Continent Challenges
Water management has always been a critical component of unconventional oil and gas development. However, a number of factors came together in recent years to make sound water management even more challenging across the Mid-Continent as well as other areas of the country.
Changes in water demand, fresh water availability, and disposal of produced water are causing many operators to evaluate their water management strategies carefully. As drilling and completion technologies have evolved, the volume of water required for each well has increased. At the same time, operators seek to reduce fresh water usage due to concerns over the sustainable use of fresh water resources. In addition, recent changes to regulations governing salt water disposal wells (SWD) due to induced seismicity concerns have made produced water disposal more challenging in some areas.
While water management challenges exist in all unconventional plays, the full range of challenges, especially concerns with induced seismicity related to SWDs, are most clearly displayed in Oklahoma. This article will focus on developing water management plans for the most active drillingand completion areas in Oklahoma. These are the South-Central Oklahoma Oil Province (SCOOP) and Sooner Trend Anadarko Basin Canadian and Kingfisher Counties (STACK) plays (see map).
Due to changes in drilling and completion practices, the average volume of water required for hydraulic fracturing of a shale well has increased in the last several years. As drilling technologies have improved, the typical length of horizontal laterals has increased from 5,000 ft. to 10,000 ft. or more. This increased lateral length increases the wellbore volume, and thus, the volume of water required to complete each well.
In addition, some operators have changed from cross-linked gel fracs to slick water fracs. Gel fracs typically carry more proppant per volume of water, but water quality and chemical requirements make gel fracs more expensive. Slick water fracs typically require 60 to 80 percent more water per well to carry the same amount of proppant. Operators have found that increasing the amount of proppant per well increases production. The increased proppant load requires a corresponding increase in the volume of water per foot of wellbore.
At the same time, operators in both plays have moved from drilling one or two wells per pad to hold acreage positions to now infill drilling their acreage positions (see graphic). During infill, multiple wells are drilled on the same well pad, resulting in less time to drill and complete each well and more wells drilled per year. Drilling activity in the STACK and SCOOP plays doubled from 2010 to 2017, with 348 wells drilled in 2010 and 704 wells drilled in 2017. This increase in the number of wells substantially increased the volume of water needed in the STACK and SCOOP plays over that time.
With the increases in lateral length, water per foot of wellbore, and the number of wells, the demand for water to perform completions has increased significantly. This rise in water demand for oil and gas has led to additional concerns about sustainable use of freshwater resources in the state of Oklahoma.
The primary source of water for both the STACK and SCOOP plays is surface water. Surface water sources include rivers and streams as well as manmade lakes and ponds. Several operators have obtained permits to withdraw water from flood control lakes or from municipal reservoirs. Groundwater sourcing is not common due to aquifers limits, but in a few areas, aquifers have sufficient capacity to support hydraulic fracturing.
Rainfall over most of the STACK and SCOOP plays ranges from 24 to 32 inches per year. While in most years this is adequate to allow withdrawals for oil and gas operations, water sources can be widely scattered, resulting in high transportation costs. In addition, due to periodic droughts and competing demand for fresh water, there is increased emphasis from regulators and the public on reducing fresh water withdrawals.
DISPOSAL AND INDUCED SEISMICITY
Disposal of produced water is a significant economic and environmental concern for all oil and gas companies. Since the 1930s, SWDs have been the preferred disposal option for both regulators and operators. Increased oil and gas production from unconventional reservoirs has resulted in significantly more water production and a concurrent increase in the water volumes injected in SWDs. For example, injection into the Arbuckle formation increased by 81 percent from 2010 to 2015.
During this same period, earthquakes in central Oklahoma with a magnitude of 3.0 or greater increased from near zero in 2010 to more than 800 in 2015. Due to a strong apparent correlation of increasing injection volumes to increased seismic activity, the Oklahoma Corporation Commission took steps to reduce injection in the Arbuckle in 2015. Since then, the state has continued to restrict disposal into the Arbuckle to a point where it is difficult to obtain a new permit to inject wastewater into that formation.
As shown, when Arbuckle injection volumes decreased, there was a corresponding decrease in the number of earthquakes. While these new restrictions appear to have been effective in reducing seismicity associated with SWDs, reduced Arbuckle injection volumes have left operators with few options for economically disposing of produced water. As a result, SWD operators have been permitting wells in shallower disposal zones, which usually have substantially less injection capacity. Operators have reported that it takes about 10 new SWDs to replace one Arbuckle well and this substantially raises the cost of disposal.
WATER MANAGEMENT PLANS
Water management plans for unconventional oil and gas operations typically have consisted of finding a water source, transferring water via aboveground pipe to an impoundment and then on to the well pad for completion. After completion, produced water was trucked to an SWD. Over time, many operators moved to water transfers via underground pipe, but the basic strategy remained much the same.
Continued restrictions on disposal into the Arbuckle make it difficult to obtain an injection permit into that formation.
While many operators can effectively produce with such a strategy, more and more operators are finding that a holistic water management plan includes recycling of produced water to round out a cost-effective strategy.
Recycling allows operators to meet their increasing per-well demand for water while decreasing overall demand for freshwater and reducing costs to acquire. Recycling also provides a more reliable supply of water that is less subject to drought and it allows operators to reduce disposal volumes in response to changes in SWD regulations or public pressure.
An effective water management plan for recycling requires a life-cycle analysis of water use and disposal for an operator’s entire acreage position. Such analysis includes an assessment of the per-well water requirements as well as analysis of drilling and completion plans to determine spatial and temporal water demands. Once analysis is complete, water management infrastructure can be designed and built.
To manage the water needed for completions, facilities for storage and treatment of produced water are needed. These facilities are sited throughout the acreage to allow economic distribution of water among facilities and to all planned wells. Each facility is connected via buried high-density polyethylene (HDPE) pipe so that water can easily be moved anywhere in the system to meet demand and to accommodate disruptions that may occur at a given facility.
Recycling facilities are typically co-located with an SWD injection well and include a tank battery to remove solids and oil from the water, along with a series of treatment and storage impoundments. Treatment impoundments allow remaining solids to settle out and include an aeration system to prevent unwanted bacterial growth.
Water storage impoundments typically have a capacity of approximately 500,000 bbl. Recycle facilities in general are built with a total of 2 MMbbl of treatment and storage capacity. Treatment systems often include specialized microbes to further reduce organics. As water moves through the system, more solids settle out and more organics are removed, so that the last impoundment contains a clean brine suitable for use in a slick-water frac.
This type of comprehensive water management and recycling system allows operators to cost-effectively respond to the continuing challenges of increased water demand, limited water availability, and reduced injec tion capacity in SWDs. While such a system requires a sizable capital investment, it typically pays for itself in three years or less, while simultaneously providing a reliable, environmentally sound supply of water to support drilling and completions.
Authored by D. Steven Tipton, Mark F. Kidder, Mark Faucher, Nate Alleman