Barnett Field in Texas is the pioneer in the shale gas production in United States. Since 1980 the Mitchell Energy Company began with pilot proofs for the production of gas and years later in 2003 by combining the horizontal wells with multiple fractures they achieve his extraction in profitable form. The maximum production was reached in December of 2011 with a production of 6 billion of cubic feet per day. The development began in Dalton county and it was spreading toward 24 counties, covering a place of 8000 square miles (net area: 5140 square miles), with a total of 19600 wells drilled and an Accumulated production of 15,6 trillion cubic feet in July of 2014.
Productive formation Barnett is located in the Fort Worth basin in the North center of Texas.
The basin is bounded to the east by the Ouachita fold and thrust belt, to the south by the Llano uplift, to the west by the Bend arch, and to the north by basement uplift arches. The basin was formed during the late Paleozoic Ouachita orogeny, a thrust-fold deformation sequence related to the formation of Pangaea.
Structural features within the basin itself include the Mineral Wells fault in the northeast, a basement fault that was periodically reactivated during the late Paleozoic. Minor normal-faults and graben blocks, present throughout the basin, locally impact basin structure and lithology. Open fractures and faults are rare within the Barnett, with nearly all fractures seen in core and logs filled with calcite. (1)
The Barnett itself is of Mississippian age, and lies on top of two Ordovician carbonate groups, the Viola-Simpson formation and the Ellenberger formation. The Viola-Simpson thins when moving NE to SW in the basin and eventually pinches out completely, leaving the Barnett directly on top of the Ellenberger. Also pinching out from NE to SW is the Forestburg limestone, a carbonate debris flow formation dividing the Barnett into upper and lower intervals.
Barnett itself thickens to the northeast, reaching a maximum thickness of more than 500 feet at depths greater than 8000 feet. (2) The figure shows the structural cuts in the West-East and North-South directions.
The Barnett shale consists of siliceous organic-rich mudstone, limestone, and minor dolomite. The shale intervals generally contain high silica (35-50%), relatively low clay (<35%), and significant organic carbon (3-10%). Thin carbonate debris flow layers are common in some areas of the basin. (3)
The first wells in Fort Worth Basin were drilled in 1982 looking for the gas production from open natural fractures, especially in areas of faults and folds associated with structurals high. The production achieved was 120Mcfg/d. In 1983 it was tested to fracture the formation with foam of C02 without obtaining great results. The next wells were drilled with Nitrogen foam.
In 1990 was introduced the Hydraulic fracture in vertical wells with the purpose to reopen and interconnect the natural microfractures present in the Barnett formation in a more efficient form and increase in this form the drainage area.
The Barnett formation is productive in the upper and lower member, both members are fractured separately, achieving a bigger efficiency and average initial Production per well of 1,050MCFG/d.
The picture shows the simulation of a typical fracture in the inferior member of the formation with a length of 2500 ft, a wide of 1 inch and a height that include the totality of the formation.
In the years 2001-2002 was carried out the first drilled of horizontal wells with multiple fractures, applying the SurgiFrac technique of Halliburton (It allows doing big fractures with a better control of the position and growth of same, by means of a loud velocity of injection).
The horizontal length was guided in perpendicular form to bigger stress direction of the formation and its length was near 2200 ft. Fractures hydraulics that are spreading in this direction induce the reopening of natural fractures in the direction of less stress.
With the progress achieved in the horizontal drill and fracture techniques the new wells reached an initial production of 2.5–3.5MMcfg/d.
Due to the change of the lithologic and Petrophysical properties along the areal variation in the formation we will find small and significant differences in the characteristics of the wells. We're going to take for the analysis of the well type, the well that is the result of the average of the total number of wells drilled in the Barnett field.
The well type has a cost of 3,5 million of dollars, a lenght of 2500-3500ft, between 8 to 12 fracture stages separated among themselves for 300ft. An average flow rate of pumping by stage approximately of 55 bpm, a water volume of 180000 barrels and 400000 pound of sand per stage.
The 95.5% of the fracture fluid is formed of water and sand, 0.5% is formed by acid, friction reduction, surfactant, gelling agent, oxygen scavenger, crosslinker, iron control, corrosion inhibitor, and antibacterial agent.
The next graphic shows the average production rate in the first 12 months of the well's life through the years.
The descent in the production rate from 2011 is principally to the exhaustion of the zones of better petrophysical properties, forcing to drilling to advance toward the zones with more poor properties.
The daily average production in the first year of the horizontal well's life during 2013 is 988Mcft/d near eight times more that the production of a vertical well that is of 120Mft/d. As a result, drilling vertical wells goes on continuous descent, becoming currently in less than 5% of the total number of wells drilled.
The next map shows the distribution of the horizontal wells, vertical, and directional at beginning of the 2014 supplied for the EIA.
The declination curves in the production of the shale gas field is pronounced, help us to understand its life cycle and the form the exploitation the same ones.
Barnett field shows an average rate of declination in the first year of 56% of the initial production, in the second year 27%, in the third year 23% and in the fourth year 20%, in this way the production declination in the first three years in the well's life in the first three years becomes 75% of initial production.
The average cumulative production in horizontal wells is 0,95 billion cubic feet (average of 12747 wells) over a lifespan average of 58 months. Just 1% of the total number of wells drilled in the Barnett (127 wells) training reached one ten-year-old life and a 14% (2156 wells) of horizontal wells that are not productive to date have shown an average life of 37 months with an accumulated production of 0,38bft.
Very few exceptional wells have overcome a cumulative production of 5 billion cubic feet.
Estimated ultimate recovery (amount of gas that produced a well in its life) varies much from one county to another in the Barnett formation, the five counties located in the "Core Area" (Tarrant, Johnson, Denton, Wise y Parker) produce near of the 92% of the total production.
The largest estimated ultimate recovery is located in Tarrant County with an EUR=2,5 billion cubic feet per well in a total of 3980 wells tested. The fewer estimated ultimate recovery is located in a group of the 19 counties with an EUR=1,01 billion cubic feet. The EUR average in the 24 counties is 1,595 billion cubic feet per well, in a total of 15153 wells.
In the first four years in the well's life it shows an accumulated production of between the 51% to the 58% of the EUR. It is to say that more the half of the production in the well's life in a shale gas field is obtained in his first four year of life.
The rate of field declination is the rate of declination that the entire field would present if not will be drilled more wells from a given time. Taking as reference year the year 2013 and only analyzing horizontal wells, the Barnett field has a rate of annual declination of 23%, being in production about 12000 horizontal wells.
The picture shows the production rate and number of horizontal wells drilled between the 2008 to the 2013 in the Barnett field.
If we know the rate of field declination and the daily average production of each well we can know the number of wells that have to be drilling each year to keep the production in the present level.
It's necessary to drill 1161 wells each year to keep the production level in 5 billion cubic feet per day. As the estimated cost is $ 3.5 million to drill a horizontal well, it is necessary to invest $ 4 billion a year to maintain the level of production.
The Current rate of drilling is near of 380 wells per year, it's below the required value, in consequence the Barnett field has a declination rate of 9% annual.
The University of Texas at Austin made a production forecast of the recoverable reserves by 2030 under different possible scenarios.
For the analysis the University took a saturation of water of 25%,a pressure of reservoirs of 3500psia, a price of $4/MMBtu, life of the well of 25 years maximum, A total area of 8000 square miles. An original gas in place of 444Tcf. A type declination curve and EUR was estimated for each zone which resulted from the analysis of the history of production of 16000 wells .
They also considered the production of dry gas, gas and condensate and oil produced in the wells of some counties to make the forecast.
In the base case (more probable), the study forecasts a cumulative 44 trillion cubic feet (TCF) of recoverable reserves from the Barnett, with annual production declining in a predictable curve from the current peak of 2 TCF per year to about 900 billion cubic feet (BCF) per year by 2030.(4)
Being necessary for its realization a total of 29.217 horizontal wells, having 15144 wells perforated to the 2010 is necessary to perforate 14073 new wells, having been drilled 3000 wells between 2012 and 2013.
Barnett field is the second shale gas producer in the United States after Haynesville field, it provide a 22% of the shale gas production and it will continue to play an important role in the country's energy grid in the future.
Barnett study determines full-field reserves, production forescat. OIL&GAS Journal. Authors: John Browning, Scott W. Tinker, Svetlana Ikonnikova, Gúrcan Gúlen, Eric Potter, Quilong Fu, Susan Hovarth, Tad Patzek, Frank Male, William Fisher, Forrest Robert. The University of Texas at Austin.
U.S. Energy Information Administration. EIA
Svetlana Presentation Morning.
Texas Railroad Commission Production Data Query System.
Barnett Shale, Resources, Reserves & Future, Production, Authors: Svetlana Ikonnikova, John Browning, Gurcan Gulen, Eric Potter, Qilong Fu, Susan Horvath, Tad Patzek, Frank Male, Ken Medlock, Likeleli Seitlheko and Scott Tinker. November, 2012. Austin Texas.
Modern Shale Gas Developent in the United States. September 2013. National Energy Technology Laboratory. http://www.netl.doe.gov/File%20Library/Research/Oil-Gas/shale-gas-primer-update-2013.pdf
World Shale Gas Resources: An Initial Assessment of 14 Regions Outside the United States. U.S.Energy Information. April 2011.
Economic Optimization. Analysis of the Development Process on a Field in the Barnett. Shale Formation. Authors, Chukwuma Uzoh, Jiahang Han, Li Wei Hu, Nithiwat Siripatrachai, Tunde Osholake,Xu Chen. 3-5-2010.
Drilling Deeper. Postcarbon Institute. Autor: J. David Hughes. October 27, 2014.
Barnett Shale: The Start of the Gas Shale Revolution Gas Shale Revolution. Author: Vello A. Kuuskraa, President. Advanced Resources International, INC. Beijing, China. April 2010.
A Comparative Study of the Mississippian Barnett Shale, Fort Worth Basin, and Devonian Marcellus Shale, Appalachian Basin. National Energy Technology Laboratory. EIA. Authors: Kathy R. Bruner and Richard Smosna. April 2011.