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The Codell Sandstone is a low-resistivity, low-contrast pay in parts of the northern Denver Basin. The area of new oil and gas production is in the deeper part of the basin between the Silo and Wattenberg fields of Wyoming and Colorado, respectively. Thickness of the Codell averages 15 to 20 ft in this area. The cause of the low resistivity is clay and pyrite content. Cores of the Codell illustrate that the sandstone is low permeability, low porosity, bioturbated and reworked finegrained marine shelf sandstone. The Codell is the upper member of the Carlile Formation and unconformably overlies the lower Carlile shales or the Greenhorn and is unconformably overlain by the Niobrara Formation. The Niobrara, lower Carlile, and Greenhorn formations are important source beds for the Codell in the Denver Basin. All are regarded as Type II sapropelic source rocks. Keys to new production are source rock maturity, horizontal drilling, and multistage fracture stimulation. Sweet spot areas coincide with high heat flow, high gas-oil ratios, overpressuring, and natural fracture development.
This article is a synopsis of paper SPE 71045, "Restimulating Tight Gas Sand: Case Study of the Codell Formation," by R.N. Sencenbaugh, SPE, D.M. Lytle, SPE, T.J. Birmingham, and J.C. Simmons, SPE, HS Resources Inc., and M.T. Shaefer, SPE, BJ Services, originally presented at the 2001 SPE Rocky Mountain Petroleum Technology Conference, Keystone, Colorado, 21-23 May.
Abstract Based on the relationship between formation pressure and minimum horizontal stress, as well as the low leak-off rate and abnormal high reservoir pressure commonly observed in tight sand, the authors devised a new method to calculate the reservoir pressure in tight sand using the pre-frac test. Numerous methods have been published to calculate reservoir pressure using pre-frac test. However, all of them require extended pre-frac testing time, taking hours in conventional formation while days in tight sand. It is not practically viable to keep the frac crew waiting so long before a frac can be executed. In contrast, this new method uses the information collected from a 5-minute pre-frac test, which is standard routine for fracs in the Denver-Julesburg basin (DJ basin). Furthermore, this new method has been validated by the conventional well tests conducted by several service companies. Introduction The Codell Formation is the primary reservoir at the Wattenberg field in the DJ basin. Due to the nature of its offshore depositional origin, the reservoir quality of the Codell sandstone has been deteriorated by high shale content and intensive bioturbation, as well as diagenesis. As a typical basin center reservoir, the Codell sandstone at the Wattenberg field possesses abnormally high original reservoir pressure, which is one of the major driving forces of well performance. Because of extended development history and intensive drilling activity, reservoir pressure depletion has been reported in many areas. It has been confirmed that well performance can be substantially affected by pressure depletion, which makes this already economically marginal formation more economically risky. Therefore, the success of infill drills largely depends on the current distribution of the reservoir pressure. Two types of commercially available well-testing methods have been applied at the Wattenberg field. One is the Pressure Express (XPT) developed by Schlumberger, and another one is the Dynamic Injection Fracturing Test (DFIT) performed by hydraulic fracturing companies. We conducted the XPT in several wells. It is the first time to utilize this type of tool in the Wattenberg field. Because the XPT test requires extended testing period in order to reach the radial flow regime in tight sand, tool stuck in open hole is always a concern. In many cases, tests had to be terminated prematurely for this issue. On matter of the DFIT tests, it usually takes two days to complete a test at one depth. In order to obtain multiple pressures in one wellbore, several days is needed before the well can be hydraulically fractured and put into production. Using pre-frac tests to calculate reservoir pressure in tight sand in the Rocky Mountain region has been published (Rajappa, 2007). This method employed the same theory as the DFIT test. Just like the DFIT test, using the pre-frac test to calculate reservoir pressure requires extended waiting time to ensure that the test reaches the radial flow regime. In order to accommodate the pre-frac test into a frac job, the frac crew usually moves in and starts the pre-frac test on Friday. The pre-frac lasts the entire weekend. The frac crew comes back on Monday to finish the frac job.
Mohaghegh, S.D. (West Virginia University) | Popa, A. (West Virginia University) | Gaskari, R. (West Virginia University) | Ameri, S. (West Virginia University) | Wolhart, S.L. (Pinnacle Technologies)
Abstract In a detail data mining study about 150 wells that have been completed in the Codell formation, DJ Basin, have been analyzed to identify successful practices in hydraulic fracturing. The Codell formation is a low permeability sandstone within the Wattenburg field in the DJ Basin of Colorado. Since 1997 over 1500 Codell wells have been restimulated. As part of a Gas Research Institute restimulation project 150 wells were studied to optimize candidate selection and identify successful practices. Hydraulic fracturing is an economic way of increasing gas well productivity. Hydraulic fracturing is routinely performed on many gas wells in fields that contain hundreds of wells. During the process of hydraulically fracturing gas wells over many years, companies usually record the relevant data on methods and materials in a database. These databases usually include general information such as date of the job, Service Company performing the job, fluid type and amount, proppant type and amount, and pump rate. Sometimes more detail information may be available such as breakers, additives, amount of nitrogen, and ISIP to name a few. These data are usually of little use in complex 3-D hydraulic fracture simulators. These models require additional and more detailed information. On the other hand, the collected data contain valuable information that can be processed using virtual intelligence tools. The process covered in this paper takes the above-mentioned data and couples it with general information from each well (things like latitude, longitude and elevation), any information available from log analysis and production data. The conclusion of the analysis is a set of successful practices that has been implemented in a particular field and recommendations on how to precede with further hydraulic fracture jobs. In this paper the results of applying this process to about 150 Codell wells during the GRI sponsored project is presented. This process provides an important step toward constructing a comprehensive set of methods and processes for data mining, knowledge discovery, and data-knowledge fusion from data sets in oil and gas industry. Introduction Patina Oil and Gas has been very active in the DJ basin in recent years. They have been one of the most active operators in the United States in identifying and restimulationg tigh gas sand wells. Patina has over 3,400 producing wells in the basin, and has restimulated over 230 Niobrara/Codell completions so far. Furthermore, it is estimated that the results they are achieving in terms of incremental recoveries are up to 60% better than other operators. Studies and analysis such as the one being presented in this paper has the potential to help operators like Patina Oil &Gas to increase their chance of success even to a higher percentage. It also has the potential to help other operators in increasing their chances of success in DJ Basin or any other locations throughout the North America. This stuy is probably one of the most comprehensive analyses of its kind ever to be performed on a set of wells in the United States. In this technical paper the authors' intention is to introduce this new and novel methodology in its entirety and present as much of the results as the page limitations of this paper allows. Please note that due to the comprehensive nature of this methodology many of the topics cannot be discussed in much detail. It is our intention to introduce these topics in much more detail in series of upcoming technical papers.
Greeley, Colorado is situated right in the heart of the DJ Basin's Wattenberg Field, in what is referred to as the " Sweetest of the Sweet Spots" for hydrocarbon production from Niobrara chalks and the Codell sandstone. While horizontal drilling has been very active all around the city of Greeley in the past few years, very little activity has occurred within the city limits, due in part to a lack of subsurface control needed for effectively and safely drilling Niobrara and Codell horizontal targets. A privately held oil and natural gas producer based in Greeley put together a plan to acquire subsurface information in order to drill horizontal wells. In early 2013 they contracted to acquire one of the largest urban 3D seismic programs in the country. Accomplishing this 23 square mile survey was no small task and required many months of extensive pre-planning by both the operator and contractor in order to complete the 3D acquisition in an urban environment. Seismic data processing employed state-of-the-art static corrections and noise-reduction technologies so as not to smear any small faults and structures at the target level. Interpretation involved detailed structural analysis and framework modeling of the Niobrara and Codell formations as understanding of the structural dips and nature of the listric faulting at the target level is paramount in placing the laterals. Additionally, seismic reservoir analysis, using Neural Network inversion technologies, identified reservoir sweet spots within the Niobrara chalks & Codell sandstone for optimizing the frac staging and completion designs. The operator is currently using the interpretation results from this 3D survey in their successful drilling program. To date, 10 extended reach horizontal wells have been drilled off this survey. The 3D survey has allowed forcorrectly targeting Niobrara and Codell objectives, in terms of staying in the hydrocarbon layer while drilling the lateral, avoid/navigate around geologic faults which could throw the well out-of-zone and impact ultimate production, and reduce the number of drilling days and drilling costs.