Injection and Seismicity Operational Risk Factors for Underground Injection Wells

Management of produced water is driving subtle yet potentially consequential changes in certain regions and underground-injection-control (UIC) formations.

UIC operation topics of interest include induced seismicity (from UIC injection, extraction, or unknown/anomolous activity), formation pressurization from years of UIC injection, and reallocation of produced water from originating “tight” producing formations to more receptive conventional, depleted, or waterflooded formations, resulting in formation pressure changes. Early signs of diminished formation health or threats to operations continuity can include

Barely felt (low-magnitude) induced and anomolous earthsquakes, which, nonetheless, are a cause of significant concern to the public and, by extension, landowners, stakeholders, and regulators.

Reduced formation injectivity, suggesting that injection rates should be dialed back or that injection-pump pressures be increased, or that water be transported further to more-receptive injection formations.

If encountered, stresses can contribute to cost overruns, impairment to operations schedules, additional oversite to assure regulatory compliance, reduced public goodwill, and a potential impairment of industry’s social license to operate.

Through analysis of publicly available UIC-operations data and well-completion data, this paper evaluate methods that might help assess UIC issues qualitatively from macro, regional, local, and subsurface-formation perspectives. The methods discussed may provide operators with a consistent simple method to identify qualitatively those assets and UIC formations with higher potential relative risk that may warrant more-detailed exploration and possible risk mitigation.

Introduction
To understand the mechanisms that may contribute to induced seismicity or impaired formation health, this paper proposes implementing simple petroleum-engineering applications to interpret available data. Data should include publicly available data (e.g., permits, completion data, and reported monthly volumes/pressures from agencies such as the Texas Railroad Commission and the Oklahoma Corporation Commission). Though public data has a number of limitations, the breadth of data allows for analysis of a state, county, or oil and gas play on a formation/depth basis or on a regional basis. Historically, this large-scale, in-depth analysis has not been performed on UIC Class II data and is beneficial in establishing long-term trends, discerning possible watch areas, and assessing formation health as it pertains to UIC operations.

Induced Seismicity. Human activity may induce earthquakes in many ways (e.g., reservoir impoundment changes, mining, fluid injection, fluid extraction, nuclear power generation, geothermal injection, and extraction). Each of these actions fundamentally causes earthquakes in the same way: They change the stress conditions on faults, which can facilitate failure. Oilfield fluid injection into saltwater disposal wells or as part of enhanced-oil-recovery programs has been shown to contribute to seismicity mainly by reducing normal stress so that movement occurs along a preexisting fault. However, a combination of many factors is necessary for injection to induce felt earthquakes. Among the required factors are the following:

  • Injection near active faults or fault systems
  • Faults that are large enough to produce felt earthquakes
  • Stresses that are large enough to produce earthquakes
  • Faults oriented suitably to stress orientation
  • The presence of fluid/pore-pressure pathways from the injection point to faults
  • Fluid pressure changes large enough to induce earthquakes
  • Faults having appropriate frictional property so that slip occurs rapidly enough to induce seismicity

Formation Pressurization. UIC pressurization issues can be divided generally into two categories: long-term formation pressurization and local, well-specific operational issues that might affect the well and near-wellbore formation conditions.

Operational practices contributing to local well-specific pressurization may include formation plugging (e.g., from injection of suspended solids, precipitation, fluid/fluid interactions, chemical interactions, hydrocarbons, bacteria, or biofilm) and flowline or tubing fouling (generally, from scale.) In most cases, the operational issues can be mitigated with effective well-maintenance programs. Remedial measures to mitigate capacity losses include the following:

  • Acidizing to remove scale (typically hydrochloric acid or hydrofluoric acid)
  • Hydraulic fracturing to increase formation permeability
  • Backflowing of wells to clean the formation face
  • Mechanical cleaning
  • Chemical additives to reduce or mitigate effects of corrosion, scale, iron, bacteria, emulsions, and surface tension or to increase formation wettability

More-challenging operational issues may be mitigated by preemptive and possibly continuous treatment of certain types of influent fluids to increase operating efficiency and extend the useful life of the UIC well. Typical systems considered will include treatments for removing skim oil and other hydrocarbons, filtration, and desilters/desanders for total-suspended-solids reduction and treatment for bacteria management, scaling, corrosion control, and iron inhibition.

Inadequate well design can be a contributing factor to localized surface-pressure increases. As injection rates increase, at certain thresholds, tubing size becomes restrictive and contributes to friction losses in the tubing, which may drive up pressure-pumping requirements

Even localized pressure perturbations, or fluctuations, may contribute to increased risks associated with seismicity under certain conditions. Therefore, avoiding pressure fluctuations or surges brought about by high-volume injection rates (either short intervals or sustained) may lower these risks. Injecting at generally steady-state rates (as market conditions allow) is preferred for this reason.

Long-term formation pressurization may occur from many years of ongoing UIC injection into closed or semiclosed systems. This is contrasted with formations that are both receiving injection and are productive (generating hydrocarbons and produced water), such as in a waterflooding operation, which can be viewed as an open system. In this case, operators typically try to achieve or maintain initial formation pressures by regulating or balancing the amount of fluids injected to achieve a stable pressure regime. These types of operation are less subject to pressurization encountered with closed systems.

The development and exploitation of shale and tight oil and gas resources adds a layer of complexity to UIC operations. Conventional oil and gas resources generally have higher porosity and permeability than shales and tight formation and thus can more-easily accept produced water. Conversely, the very low permeability and porosity of shale and tight plays does not accommodate produced-water injection efficiently or economically, with very low overall injectability. It is not unusual for operators to reallocate produced water from unaccomodating shale and tight plays to older depleted conventional reservoirs that are more receptive. Over time, this may reduce pressure in the unconventional formations while increasing relative volumes, and potentially pressures, in the more-receptive conventional formations.

Contributing factors for seismicity and well pressurization are different from each other, but some common elements are shared. Thus, by assessing some of the common elements, namely related to pressures, volumes, and pressure/volume relationships, one can qualitatively assess partial risk profiles for both conditions, provided that other unique contributing factors are assessed in addition.

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