GSA Critical Issue: Induced Seismicity

Introduction

(continued)

In the past decade, North America has experienced a boom in oil and gas production[1] from unconventional reservoir rocks. The term “unconventional” relates to low permeability reservoir rocks (such as shale) that may contain significant oil and gas but do not readily yield hydrocarbons. However, advancements in drilling technology allow companies to drill vertically, then turn and drill laterally for thousands of feet, and then hydraulically fracture (frack) the rock to release the oil and gas. These lateral, near-horizontal well bores are targeted to align with the attitude and structure of a geologic formation that holds tremendous amounts of trapped oil or natural gas.

Horizontal drilling coupled with hydraulic fracturing uses water to transfer the hydraulic pressure needed to fracture the target rock. This water typically contains minor amounts of chemical additives and treatments (normally less than 5%) to transfer hydraulic forces more effectively (e.g., surfactants), as well as to enhance productivity (e.g., acids, biocides) in the target formation. Modern hydraulically fractured wells may require over 3 million gallons[2] of water for treatment. A by-product of both conventional and unconventional oil and gas production is formation water that is brought to the earth’s surface along with hydrocarbons—much of it in the form of brackish or concentrated brines. This saltwater waste, plus the flowback water from the hydraulic fracturing process, is either treated and recycled for further hydraulic fracturing, or handled as waste water needing disposal.

For decades, oil and gas producers have looked for economic ways to recycle, treat, reuse, or dispose of the wastewater. One common way is to recycle and reuse the wastewater for additional hydraulic fracturing. Sometimes water is re-injected into conventional oil fields to create a “water flood” where water, being a more dense liquid than crude oil, will force any remaining oil toward designated extraction wells. This water-flooding process is referred to as secondary recovery. If reinjecting water is ineffective in forcing additional hydrocarbons out of the reservoir, tertiary treatments, such as injecting carbon dioxide, may be used to further stimulate production. In other situations, the wastewater has no further use and is injected for permanent disposal into acceptable rock formations either deeper or shallower than the producing zone. The disposal of high volumes of fluids, over long periods of time, can alter the pressure conditions in the subsurface, potentially causing existing faults to slip, leading to seismic events (earthquakes). These earthquakes can be detected by instruments, or in some cases felt on the land surface in the vicinity of earthquake’s location. In rare cases, motion may be felt up to hundreds of miles away from the disposal wells[19]. To date, felt earthquakes related to disposal wells are not the norm and appear restricted to vulnerable regions. In Texas, there are approximately 7,500 active disposal wells and only a small number of reported cases of induced seismicity. However, some larger seismic events have been sufficiently powerful to cause moderate damage to buildings and structures on the land’s surface near the injection sites[4].

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