The BVI-PERM Plot
Petrophysical Answers for Bossier Sandstones
East Texas Basin
A presentation to the Houston geological Society''s North American Explorationists group on March 22, 2004.
Speaker: Rick Turner, Barrow-Shaver Resources
Abstract: The Bossier Sand is a primary drilling objective for natural gas on the west flank of the East Texas Basin. The sand is upper Jurassic and was deposited during Kimmeridgian and Tithonian time. The production is usually found at depths between 10,000 feet and 16,000 feet, with production tests ranging from one million to twelve million cubic feet of gas per day. The sand is tight and requires fracture stimulation to produce at economic rates.
Formation evaluation of gas bearing zones in the Bossier in terms of producibility is a petrophysical challenge. Reading the porosity from the log and calculating the water saturation will not yield much information concerning the producibility of the rock. Cross-plotting methods are necessary to make meaningful determinations. The porosity values from the logs must be lithology corrected, borehole corrected, shale corrected, and corrected for gas effect to establish effective porosity values. The water saturation should be calculated with a saturation model that accounts for shale volume such as Fertl and Hammack or Simandoux. Since the gas bearing Bossier sands usually produce water free, they are assumed to be at irreducible water saturation.
The Bulk Volume water Irreducible – Permeability (BVI-PERM) cross-plot is generated by plotting water saturation on the horizontal axis and effective porosity on the vertical axis. Lines of constant bulk volume water irreducible (BVI), and lines of permeability are drawn on the plot and used as a template. The permeability lines are derived from the Coates (1977) permeability equation that relates permeability to porosity and irreducible water saturation. Data points plotted on the graph fall within a grid created by the BVI and permeability lines. The position of the points within the grid yields interpretative information about the producibility of the formation.
At irreducible water saturation, lower values of BVI are related to larger grain size, and better permeability. For example, a well with a given porosity with a high BVI will be a poor producer, while another well with the same porosity and a low BVI will be a good producer due to larger grain size and better permeability. Such subtle differences are not visible when looking at the water saturation and porosity alone.
In addition to the BVI-PERM cross-plot, a producibility index utilizing porosity-derived permeability, formation pressure, and formation temperature can be calculated. When the producibility index is plotted against the average BVI for porosities above the 10 md Coates permeability line, an estimate of the ultimate recovery for a well can be made.
The methods for analysis of the Bossier basically rely on amplifying the porosity values through the Coates permeability function and then comparing them to their corresponding water saturations through cross plotting. The producibility index adds the dimension of formation pressure and temperature to the analysis, and provides a basis for comparing the productive potential for wells in differing pressure environments. Although the cross-plotting methods are not complicated, they provide a powerful method of organizing borehole data into a decision-making apparatus that ranks wells bases of their petrophysical character.
Biography: Rick Turner, a native of East Texas, graduated from Stephen F. Austin University in 1973 with a B.S. degree in Geology and from Texas A&M University with a Master of Science Degree in Geology in 1977. Rick started with Gulf Oil Company in Houston as an Exploration Geologist working the Mesozoic of the Gulf Coast. He then worked for Fina Oil and Enron Oil & Gas in Tyler, Texas. Since 1998, he has worked at Barrow-Shaver Resources Company as an Exploration and Production Geologist.
Mr. Turner’s experience includes exploration in the Smackover of South Arkansas and East Texas, the Paluxy of East Texas, field development of the Cotton Valley in East Texas, discovery of new reserves beneath the salt overhang at Minden Salt Dome in Webster Parish in North Louisiana, and the successful development of by-passed reserves at Oletha Field in Limestone County, Texas.
Mr. Turner’s publications include articles on the environmental interpretation of the Subclarksville and Woodbine at Kurten Field in Brazos County, Texas; low-resistivity production from the Smackover in North Louisiana; a theory for the development of salt dome overhangs in the East Texas Salt Basin; the occurrence of low-resistivity production in clay-coated grain facies of the Travis Peak and Cotton Valley Formations of East Texas; and an interpretation supporting Jurassic-aged strike-slip faulting in Northeast Texas.