Multiple gamma emitting tracers and post fracture spectral gamma ray logs were used to optimize production and improve the completion designs of 150 gas wells in the southwest Kansas region of the Hugoton Field. The information from the tracers and logs has revealed unstimulated pay zones and has been the impetus for completion modifications, yielding substantial gains in production. Through the use of limited stress barriers and permeability variations, perforation schemes have been successfully modified to improve fracture containment and proppant placement over the 200 ft Chase Group intervals. Previously these intervals were treated with multiple individual zone treatments. Introduction Stretching from Southwest Kansas through Oklahoma and into the Texas Panhandle, the Hugoton field is the largest gas field in North America. Gas is produced from depths of approximately 2400–2900 ft in the Chase Group with reservoir pressures of 100–200 psi. The pay zones consist of dolomite and limestone separated by siltstones and are typically stimulated to produce at economic rates. The average production rate within the field is 250 thousand Cubic Feet per Day (MCFD) per well. The Hugoton field was discovered in 1927. Until the early 1960s, drilling and setting casing at the top of the Chase Group was commonly practiced in the Hugoton field. The wells were then cable drilled through the pay interval, acidized with 10,000 to 30,000 gallons of acid and cased with an uncemented, slotted liner. During the 1960s, many of the wells were hydraulically fractured for the first time with a "150 frac", which consisted of 150,000 gallons of river water and 150,000 pounds(#) of river sand pumped at 150 Barrels Per Minute (BPM). Some of the fracture treatments were pumped as high as 300 BPM. Sand concentrations typically ramped from 0.5 to 1.5 pounds per gallon (ppg). The late 1980s saw fracture treatments increase in size to 300,000 – 500,000 pounds of sand and 80,000 – 90,000 gallons of crosslinked fluid at rates between 40 – 75 BPM. Some treatments pumped over 1,000,000 pounds of sand. During Mobil's 1991 infill drilling program, limited entry perforating techniques were used in hopes of treating the entire 200 ft Chase interval with one fracture treatment. The Chase Group was perforated 1 shot every 5 to 10 feet. From the wells stimulated with limited entry perforating, tracer surveys suggest the fractures grew into the lower stress, higher permeability zones and did not effectively treat the low permeability zones needing the fracture stimulation. The low surface treating pressures indicate the friction pressure necessary to open all the zones was never achieved. Treatment sizes were 90,000 – 120,000 gallons of 30# crosslinked gel and 400,000 – 500,000 pounds of 12/20 sand pumped at 60 BPM. Of the 29 wells drilled by Mobil in 1991, only 4 are currently producing more than the average surrounding well. P. 223
Multiple gamma emItting tracers and post fracture spectral gamma ray logs were used to optimize production and improve the completion designs of 150 gas wells in the southwest Kansas region of the Hugoton Field. The information from the tracers and logs has revealed unstimulated pay zones and has been the impetus for completion modifications, yielding substantial gains in production. Through the use of limited stress barriers and permeability variations, perforation schemes have been successfully modified to improve fracture containment and proppant placement over the 200ft Chase Group intervals. Previously these intervals were treated with multiple individual zone treatments. INTRODUCfION
Chemical frac tracing is used to evaluate flowback and cleanup efficiencies. The technique utilizes a family of unique, environmentally-friendly, fracturing fluid compatible chemical tracers to quantify segment-by-segment recovery for individual fracturing treatments and stage-by-stage recovery for multi-stage fracturing treatments. These chemical tracers with their unique chemical characteristics are mixed at a known concentration into frac fluid stages as the frac fluid is pumped downhole. Upon flowback, samples are collected and analyzed for tracer concentration. With the use of the mass balance method the flowback efficiency for each stage is calculated. These precise flowback calculations yield a more accurate assessment of cleanup efficiency. This paper presents several case histories in which the technique was implemented. Results and fracture flowback prognoses are presented. The results are also used to assess post-frac performance as a function of flowback efficiency. Introduction Chemical Frac Tracers Chemical frac tracers, CFT's, are used to precisely calculate flowback and hence flowback efficiency and to evaluate fracture cleanup. Various chemical tracers with unique chemical characteristics are mixed at a known concentration and injected according to a pre-determined design throughout the frac fluid stages, such as the pad and the propping laden fluid stages. The characteristics of these tracers are unique. They do not react with each other, the formation or the tubular. They do not degrade with temperature or time, do not self-concentrate, and do not react with frac fluids. These tracers are detectable at low concentrations of 50 ppt (parts per trillion). They are also environmentally safe to handle, pump downhole and dispose. They are soluble in water, and unlike polymers, do not concentrate upon leakoff. Upon flowback, samples are collected and analyzed for tracer concentration. With the use of the mass balance method, flowback of each frac fluid stage is calculated, and hence flowback efficiency for each stage of frac fluid injected. The precise flowback calculation for each frac fluid stage yields the fracture cleanup efficiency. Chemical frac tracing can also be used as a means to understand vertical communication between zones. Under this scenario, one chemical tracer is injected into each zone. Each zone is flowed back individually after a period of shut-in allowing the flow of fluid within the formation. Samples are collected and analyzed for tracer concentrations. If zones are communicating vertically under static conditions, the tracer from one zone can flow into other zones and hence will be detected accordingly.
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