Adding To The Geothermal Energy Portfolio From Deep Sedimentary Basins

Richard J. Erdlac, Jr.^1,2, Robert C. Trentham¹, Linda Armour¹, Robert Lee³, and Bruce Miller^4
1UTPB-CEED
²Now at Erdlac Energy Consulting
³Lee Engineering
⁴Endurance Exploration

As a state, Texas has not lacked for energy exploration and production. Oil and gas has led the way for local and state wide economic development, being an active part of state commerce for nearly 100 years. Similarly, coal has also been produced though it is generally relegated to the eastern part of the state. Over the last decade or two renewable energy resources have in the planning and development stage, with Texas more recently bypassing California as the Nation’s leading wind producer. Other renewable resources, including hydroelectric, solar, biofuels, and biomass are receiving various levels of support with increasing interest in their development. Geothermal, while in use for environmental heating and cooling, has lagged behind for electrical production due to a perceived lack of a viable resource for electrical power generation. However access to subsurface rock strata with high heat and large quantities of still inplace brines represent an untapped energy resource that the existing oil and gas industry can take advantage of in developing along with the remaining inplace oil and gas resources.

In Texas, at least five regions of deep sedimentary rock and hot temperatures (>240^oF) exist that have the potential for electrical power development. These include the Gulf Coast region, East Texas, the Delaware-Val Verde Basins, the Anadarko Basin in the Texas Panhandle, and the Trans-Pecos region. Three of these regions, Gulf Coast, Delaware-Val Verde, and Trans- Pecos, are discussed in this presentation.

During the 1970’s-80’s, the DOE invested nearly $200 million into investigating the potential for geopressured-geothermal production along the Texas and Louisiana Gulf Coast. Three forms of energy were considered: 1) chemical from dissolved methane; 2) thermal from the hot brine over 225^oF; and 3 mechanical from high brine flow rates (>20,000 bbls/day – 583 gpm) and high wellhead pressure. Only the first two forms of energy conversion were achieved. Most of the wells used for investigation were provided by the oil and gas industry, though the DOE drilled several wells of their own, with one well in Texas becoming the site of the first geopressured-geothermal power plant. The Pleasant Bayou #2 drilled to over 16,000 feet into the Frio Sandstone. It had a maximum tested flow rate of 25,000 bbls/day at a reservoir temperature of 302^oF, a wellhead temperature of 292^oF, and 22 SCF gas/bbl water. However the plant operated on only 10,000 bbls/day, producing an output of 1.191 MW of electrical power split between 541 kW from a binary cycle turbine and a gas engine generating 650 kW. With a parasitic load of -209 kW, a total of 3,445 MWh was produced and sold to a local distributor. The project demonstrated that geopressured-geothermal power production was possible from deep wells drilled in the Gulf Coast.

Work in the Delaware-Val Verde Basins expanded upon earlier work by Swift and Erdlac while operating the West Texas Earth Resources Institute in Midland. A federal and state grant provided to UTPB-CEED allowed the earlier temperature-depth (T-d) data base to be expanded to over 5,000 wells and over 8,000 T-d points. Very preliminary maps and various graphs were developed to understand the distribution and range of temperatures that might be expected in the deep formations of this basin complex. For example, temperatures of around 300^oF were found at depths between 18,000 to 19,000 feet in Pecos County. Calculated thermal gradients for the counties studied (Loving, Reeves, Pecos, and Terrell) using empirical data suggest logarithmic distributions rather than standard linear functions for T-d readings from log header information. The Trans-Pecos region has a predominance of volcanic and igneous rock exposed at ground surface making this region similar to areas in other western states. More recent deformation in the region is related to Rio Grande rifting starting in northern New Mexico and southern Colorado and trending south into this region of Texas. Higher levels of heat flow have been observed in this region. Data from a number of oil and gas wells was combined with data from the SMU Geothermal Laboratory to obtain and initial picture of the t-d regime for the Trans-Pecos.

The oil and gas industry has drilled on the order of 600,000 wells to various depths in onshore Texas play and prospect development. If only a few percent of these wells are converted to geothermal energy production wells, estimates on the value of the inplace energy is on the order of 10’s of billions of dollars. As geothermal energy is a renewable resource when managed properly, this dollar value represents an amount that can be produced every year for an indefinite period of time. Thus the importance of geothermal energy from sedimentary basins should not be overlooked by the oil and gas industry, which presently wastes this heat just as it once wasted natural gas by venting it to the air.

AAPG Search and Discovery Article #90089©2009 AAPG Southwest Section Meeting, Midland, Texas, April 26-29, 2009