Environmental Responsibility Drives Siting of Saltwater Disposal Wells

Continued oil production will increase the demand for saltwater disposal methods
By Len Davisson and Mark Luther | June 24, 2013

The Bakken, well-known for its rich petroleum deposits, has been experiencing steady oil production since 2004 when the application of horizontal drilling technologies and hydraulic fracturing facilitated oil extraction from previously unviable deposits. Generally, three products are extracted from a typical North Dakota oil well during production: crude oil, natural gas and produced formation water (saltwater). Both crude oil and natural gas are profitable, however, saltwater is not. In many parts of the Williston Basin, these saline formation waters can contain up to 300,000 parts per million of various types of salts, nearly equal to the waters of the Dead Sea and almost 10 times that of ocean water. 

Operators are required to properly dispose the high-saline saltwater in an environmentally responsible manner as high-saline waters can destroy soil productivity or pollute near-surface freshwater aquifers. Saltwater is typically disposed in one of two ways: 1) re-injected into the oil-producing formation for enhanced or secondary oil recovery, or 2) injected into an underground formation that contains a natural saline aquifer. The second method is termed saltwater disposal (SWD), and shows a high potential for profit by the disposer—if the disposal (injection) well is sited in a good location. 

Siting Factors
Locating an appropriate site for a new SWD well includes a robust set of criteria. Kadrmas, Lee & Jackson a multidisciplinary engineering and planning firm, considers several factors essential for a properly sited well, depending on the preference for a specific geographic location. Major siting factors utilized include:

Current and Future Production. Depending on client parameters, SWD sites can be located in areas where producing oil wells have low water production but are densely spaced, or in areas where producing wells are widely spread apart but have a higher water-to-oil ratio. 

Aerial Photos. A visual overview of the proposed site is assessed, typically utilizing Google Earth or similar aerial images. Geographic areas to avoid are visually assessed, such as areas with underground seepage or sites within a close proximity to homesteads and farms. Certain commercial/industrial zones may raise or lower the siting criteria as well, especially if the zone shows signs of recent expansion. 

Accessibility. A significant factor is proximity of the site to a paved road. Although access from a main corridor may seem preferable, there are growing constraints concerning additional access points along some state highways and county roads. For some situations, designing access to a SWD site involves locating egress points along county gravel roads in order to mitigate the need for an additional intersection.

Topology: Typically following the visual inspection is a topographical assessment, which considers such factors as the amount of earthwork needed to develop a site large enough to accommodate up to 30 commercial vehicles, potential interference with natural drainage patterns and spillage containment requirements.

Competition. North Dakota SWD sites typically inject saltwater into the Dakota Formation. However, because the Dakota Formation varies in thickness and consistency, the proximity of neighboring SWD sites can be a concern. The closer sites are to each other, the higher the chance that injected saltwater from one site may begin to communicate or interfere with another site’s injection potential. Proper geographic spacing of SWD sites take into consideration the current and planned locations of SWD sites, historical average of daily volume injected and the pressure needed to inject that volume.

Avoidance of Fresh Water Aquifers. Because the Dakota Formation is located above the Bakken Formation and several thousand feet below the surface, an SWD injection well must be sited to avoid penetrating overlying geological zones containing fresh water aquifers used for human, livestock or other consumptive uses. Currently, the North Dakota Industrial Commission requires a SWD well-bore to avoid any freshwater aquifer by at least a quarter-mile, or 1,320 feet.

Future Impacts

 For most producing oil and gas wells, the percentage of saltwater increases over the well’s lifetime. For example, many new wells can initially produce one barrel of saltwater for every two barrels of oil produced. However, over the well’s lifetime, that ratio can grow to 10 barrels of saltwater for every barrel of oil. This creates a situation where operators could be forced to plug and abandon a well due to rising costs of available saltwater disposal sites.

Earlier this year, Lynn Helms, director of the North Dakota Industrial Commission’s Department of Mineral Resources, said North Dakota is currently injecting 468,000 barrels of saltwater a day into the Dakota Formation. He further stated that North Dakota could require approximately 1,600 additional SWD wells, which currently can be built for $2 million to $3 million per well. For the next 50 to 60 years, these wells could potentially inject roughly 10,000 barrels of saltwater a day, at an economic return of $1 per barrel.

Authors: Len Davisson and Mark Luther
Kadrmas, Lee & Jackson Engineering
Phone: 701 355 8400
Email: Len.davisson@kljeng.com

 

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