Applications of Underbalanced Drilling Reservoir Characterisation for Water Shut Off in a Fractured Carbonate Reservoir—A Project Overview
Abstract: TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractFollowing successful implementation of underbalanced drilling in Petroleum Development Oman (PDO) an evaluation of the value of underbalanced drilling (UBD) in a northern carbonate field was commissioned. The study showed that there was a production upside to developing the reservoir with UBD. A four-well trial campaign was conducted between June and October 2004 to assess the potential.The trial campaign was implemented following a structured program built f… Show more
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Cited by 7 publications
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Underbalanced Drilling Experience in PDO
In July 2004 a Northern Oman Carbonate reservoir asset team embarked on an Underbalanced Drilling (UBD) campaign after a detailed feasibility study. The main objectives of the UBD campaign were to improve the Productivity Index (PI) and test the validity of real time reservoir characterizations (RTRC) in UBD operations. To date 16 wells have been successfully drilled UBD resulting in almost double the PI and better initial well production with a decline trend better or comparable to the offset conventional wells in the area. Real time reservoir characterizations (RTRC) helped identifing the features contributing to water production and enabled real time decision making to optimize well path and sometimes extension of the well length by correlating real time resistivity data with real time production. RTRC also enabled optimum completion selection to maximize production as well as ESP sizing and installation of proper water shut off mechanisms (the preferred water shut off method is expandable Open Hole Clad "OHC"). Reservoir and geological models were updated in accordance with this analysis. Other benefits seen from the UBD campaign include eliminating the need for stimulation, TLC logging (i.e. MPLT, FMI, etc...) and curing losses while drilling the reservoir section along with technology maturation of improved Down Hole Isolation techniques and the running of the OHC in UB situation. A key success factor was the assimilation of a multidisciplinary integrated team comprising of the asset team, well engineering, new technology, drilling and UBD contractors. They all contributed to the success of UBD campaign which resulted in new oil delivery ahead of time and within budget. As a result of the high success of the UBD campaign in 2004/2005 the asset team decided to drill all its oil producers underbalanced. However, the challenges that have arisen are mainly in the cost of UBD packages and services as well as improving the reliability of some of the well control tools such Down Hole Isolation Valves (DHIV) and the application of contingencies in case of their failure. Introduction to the field The field was first developed using widely spaced vertical wells. As the technology developed and field understanding grew horizontal wells were introduced in the early 1990's. Increased drilling activities have raised the gross fluid and net oil production rates in the field. The Northern Oman Carbonate reservoir field is located in NW Oman and is 26 km long, 12 km wide. The Reservoir forms a low relief anticline with NE-SW orientation with an overlying shale caprock. The Reservoir was deposited in a varying range of platform interiors, platform margins, slopes and basin environments resulting in a heterogonous reservoir characteristic. The reservoir is crossed by two major sets of faults. Seismic interpretation and FMI readings clearly indicate a NE-SW set and a NW-SE set. These faults act as water conduits when intersected during drilling resulting in early water break through and significant increase in BS&W. Maximum reservoir thickness is about 50 m with a large transition zone to water, the average log saturation shows the 50% water saturation level at approximately 1472 m tvdss and free water level (FWL) at 1490 m tvdss. For optimum drainage results the standard well path is designed to be three meter below the top of reservoir. Field production started in 1970 and progressively increased to reach peak production in 1973. After 1973 the production went through a gradual decline before it stabilized in the 1980's then slowly rising in the 1990's. In 2002 the production increased to reach its highest peak after 1973. Well performance in this reservoir is variable because of the heterogeneity of the reservoir facies. Field production is generally characterized by high initial gross production and rapid water cut development. Water production, in the reservoir, increased to 55% after the first four years of production. The average field water cut has been around 90% since 1982. In 2004 a prescreening study was conducted and identified this field as a high potential, low risk candidate for UBD.
Underbalanced Drilling Experience in PDO
In July 2004 a Northern Oman Carbonate reservoir asset team embarked on an Underbalanced Drilling (UBD) campaign after a detailed feasibility study. The main objectives of the UBD campaign were to improve the Productivity Index (PI) and test the validity of real time reservoir characterizations (RTRC) in UBD operations. To date 16 wells have been successfully drilled UBD resulting in almost double the PI and better initial well production with a decline trend better or comparable to the offset conventional wells in the area. Real time reservoir characterizations (RTRC) helped identifing the features contributing to water production and enabled real time decision making to optimize well path and sometimes extension of the well length by correlating real time resistivity data with real time production. RTRC also enabled optimum completion selection to maximize production as well as ESP sizing and installation of proper water shut off mechanisms (the preferred water shut off method is expandable Open Hole Clad "OHC"). Reservoir and geological models were updated in accordance with this analysis. Other benefits seen from the UBD campaign include eliminating the need for stimulation, TLC logging (i.e. MPLT, FMI, etc...) and curing losses while drilling the reservoir section along with technology maturation of improved Down Hole Isolation techniques and the running of the OHC in UB situation. A key success factor was the assimilation of a multidisciplinary integrated team comprising of the asset team, well engineering, new technology, drilling and UBD contractors. They all contributed to the success of UBD campaign which resulted in new oil delivery ahead of time and within budget. As a result of the high success of the UBD campaign in 2004/2005 the asset team decided to drill all its oil producers underbalanced. However, the challenges that have arisen are mainly in the cost of UBD packages and services as well as improving the reliability of some of the well control tools such Down Hole Isolation Valves (DHIV) and the application of contingencies in case of their failure. Introduction to the field The field was first developed using widely spaced vertical wells. As the technology developed and field understanding grew horizontal wells were introduced in the early 1990's. Increased drilling activities have raised the gross fluid and net oil production rates in the field. The Northern Oman Carbonate reservoir field is located in NW Oman and is 26 km long, 12 km wide. The Reservoir forms a low relief anticline with NE-SW orientation with an overlying shale caprock. The Reservoir was deposited in a varying range of platform interiors, platform margins, slopes and basin environments resulting in a heterogonous reservoir characteristic. The reservoir is crossed by two major sets of faults. Seismic interpretation and FMI readings clearly indicate a NE-SW set and a NW-SE set. These faults act as water conduits when intersected during drilling resulting in early water break through and significant increase in BS&W. Maximum reservoir thickness is about 50 m with a large transition zone to water, the average log saturation shows the 50% water saturation level at approximately 1472 m tvdss and free water level (FWL) at 1490 m tvdss. For optimum drainage results the standard well path is designed to be three meter below the top of reservoir. Field production started in 1970 and progressively increased to reach peak production in 1973. After 1973 the production went through a gradual decline before it stabilized in the 1980's then slowly rising in the 1990's. In 2002 the production increased to reach its highest peak after 1973. Well performance in this reservoir is variable because of the heterogeneity of the reservoir facies. Field production is generally characterized by high initial gross production and rapid water cut development. Water production, in the reservoir, increased to 55% after the first four years of production. The average field water cut has been around 90% since 1982. In 2004 a prescreening study was conducted and identified this field as a high potential, low risk candidate for UBD.
Determination of Water–Producing Zones While Underbalanced Drilling Horizontal Wells—Integration of Sigma Log and Real–Time Production Data
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe paper presents a novel logging approach used to identify water producing zones while under-balanced drilling (UBD) horizontal wells. This approach will work in mixed-wet reservoirs and is particularly attractive for mature reservoir undergoing waterflooding.The logging process consists of a standard logging-whiledrilling (LWD) data acquisition followed by a pulse neutron capture run (PNC). The PNC data is acquired after pumping the tool inside the drillpipe to total depth (TD). The LWDresistivity and PNC-sigma measurements are used to calculate completely independent oil saturation curves. This additional information is important for non-water-wet reservoirs because a separation between the resistivity and sigma-derived oil saturation will indicate a flushed zone. The separation is due to what is called resistivity hysteresis and is well documented in the literature and observed in the laboratory for this reservoir.The logging information is complemented with real-time production data which provides a pseudo-inflow log of water and oil inflow intervals along the horizontal wellbore. The pseudo-inflow log is used to validate if saturation differences correlate with water production.The approach is made possible because the wells are drilled under-balanced meaning that, during the drilling process, the reservoir pressure is always higher than the pressure in the wellbore. This pressure differential avoids drilling fluid invasion into the formation, which is necessary for shallow nuclear logging and allows the reservoir to flow for real-time production characterization.Results from eleven UBD wells show that a positive correlation exists between inflow of water and separation between the resistivity and sigma-derived oil saturation. This is in agreement with the resistivity hysteresis concept.This application can become important for projects developing mixed-wet reservoirs especially during the infill drilling or waterflood phases. The PNC logging operations takes between 6 and 12 hours and can be adapted with minimal incremental cost to most UBD operations.
Use of New Rate-Integral Productivity Index in Interpretation of Underbalanced Drilling Data for Reservoir Characterization
The prospect of dynamic reservoir characterization using flow and pressure data gathered during underbalanced drilling (UBD) is a powerful driver for implementation of UBD. The mathematical aspects of this complex, ill-posed, inverse problem have been the subject of research in the past decade. This paper focuses on practical, field implementation of UBD reservoir characterization, and the problems that consequently arise. Interpretation of data from UBD is made difficult by transducer errors, operational transients, and noise in data. It is therefore often very difficult to interpret the reservoir characteristics from the instantaneous productivity index (PI). In this paper, we introduce a parameter known as the Rate Integral Productivity Index (RIPI), which borrows from the theory of rate-transient analyses. The mathematical and physical basis of RIPI and its relationship to the instantaneous PI are presented. The behavior of RIPI and its implications for reservoir characterization are discussed. RIPI de-noises the data, and scales the problem such that the trends in data are more obvious, enabling robust interpretation of UBD data, and increasing the confidence in calls made regarding reservoir characteristics. Application of RIPI to field data is illustrated through several examples. Data acquisition, processing, and preparation for UBD reservoir characterization are discussed. In particular, the importance of filtering, de-noising, and identifying and excluding operationally induced transients is described. Limitations imposed by the data gathering methods are highlighted. It is shown that the ability of RIPI to reduce noise in raw PI data allows trends to be read more easily. The use of RIPI for static and dynamic characterization of super-matrix features (such as fractures, thief zones, etc.) is illustrated. The limitations of the approach and future trends are discussed. Introduction When drilling underbalanced in permeable reservoir rock, the return fluid carries reservoir fluids and hence, it is presumed, a signature of the reservoir being drilled. Moreover, the signature is available while drilling, which allows discrimination between progressively exposed reservoir sections. If unraveled, this signature gives us information about the reservoir that is otherwise not easily available with the immediacy with which UBD operations provide it. The prospect of unraveling this signature and thus improving reservoir knowledge is increasingly seen as an important driver for application of UBD. During UBD, pressures and rates at the inlet (injection into drill string and/or concentric gas injection annulus between casing and tie-back) and outlet (choke) are usually measured. Downhole conditions (temperature, flowing wellbore pressure) are also often captured while drilling. In general, all of these measurements are time-varying. If the inverse problem is formulated properly, these known parameters can deliver the pore pressures, permeability and other production related characteristics of the reservoir. This can have significant benefits in reservoir characterization, production optimization, and in justification of UBD. In particular, the ability to characterize the reservoir during drilling enables the Petroleum Engineer to make immediate use of the knowledge in designing completions that optimize the performance of the well being drilled. Additionally, the evaluation and exploitation of opportunities can occur almost simultaneously, with powerful implications for decision making and maximizing asset value.
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