Acid Stimulation of Extended Reach Wells: Lessons Learnt From N'Kossa Field

Mazel, Jean-Michel; Poitrenaud, Hugues; Mbouyou, Patrick Edmond Michel

doi:10.2118/107760-ms

Cited by 5 publications

(1 citation statement)

References 18 publications

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“…In some cases, a combination of pumping techniques and diversion mechanisms is implemented to achieve the desired outcome. For instance, Mazel et al (2007) showed how a combination of CT pumping and annular bullheading, along with the use of ball sealers, was necessary to efficiently stimulate an extended reach well. Under similar conditions, Jorden et al (2011) preferred the sole use of CT, along with a specially designed jetting nozzle, which was reciprocated in front of the interval to be stimulated.…”

Section: Introductionmentioning

confidence: 99%

An Innovative Methodology for Real-Time Control and Optimization of Dual Injection Increases Efficiency of Carbonate Stimulation in Horizontal Openhole Water Injectors

Ramondenc

Burgos

Delgado

et al. 2015

Day 2 Mon, December 07, 2015

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An innovative methodology using a new bottomhole flow measurement tool and distributed temperature surveying was used to treat two openhole carbonate water injectors using coiled tubing (CT) equipped with fiber optics. The system capabilities were leveraged to control, in real time, the dual injection process and ensure that the treatment covered the entire horizontal section. The new level of efficiency of this approach was proven in the outstanding results of this stimulation campaign.The method takes advantage of the fiber optics present in a CT string to acquire, in real time, both downhole flow measurements and distributed temperature surveys to identify areas of higher and lower injectivity along the wellbore. It also enables clear and rapid evaluation of the depth to the level that bullheaded fluid would normally reach downhole. The real-time flow measurement is further used to optimize the stages of the treatment that aim at placing fluid across the interval initially poorly covered, as it gives direct feedback of the efficiency of the pumping through the annulus and CT.Matrix stimulation in water injector wells is often handled in a straightforward way by simply bullheading the treatment from surface. Used within the context of horizontal or highly deviated openhole wells, this frequently leads to an overstimulation at the casing shoe or along the first part of the zone of interest, and a significant section of wellbore is poorly covered or even remains untreated. The new approach not only enables visualizing where the treatment fluid goes, it also gives a new level of control of downhole injection, in real time. This new capability was taken advantage of within the context of long horizontal openhole water injectors, whose previous stimulation treatments left untouched a significant portion of the reservoir, even when complementing bullheading by the placement of treatment fluid with CT. The methodology enabled clear identification of which pumping method was beneficial to which interval. It also proved very efficient, leading to an increase in the injectivity index of the wells of nearly threefold, beyond the expectations based on previous stimulation interventions.The new downhole flow measurement tool and its use within this innovative workflow address many limitations usually encountered during the stimulation of horizontal or highly deviated openhole water injectors. In particular, the approach enables taking full advantage of the dual injection process, which often proves difficult to control and optimize in real time. The end result is an unprecedented level of efficiency of the process.

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Section: Introductionmentioning

confidence: 99%

An Innovative Methodology for Real-Time Control and Optimization of Dual Injection Increases Efficiency of Carbonate Stimulation in Horizontal Openhole Water Injectors

Ramondenc

Burgos

Delgado

et al. 2015

Day 2 Mon, December 07, 2015

View full text Add to dashboard Cite

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Integrated approach to geomodelling and dynamic simulation in a complex mixed siliciclastic–carbonate reservoir, N'Kossa field, Offshore Congo

Wonham

Cyrot

Nguyên

et al. 2010

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The Lower Sendji Carbonate (Albian age) of the N'Kossa field is a mixed siliciclastic–carbonate reservoir exhibiting very heterogeneous reservoir properties and the development of extensive vertical flow permeability barriers. The reservoir is a succession of interstratified dolomite, limestone, sandstone and shale lithologies of sabkha, tidal flat and lagoonal origin. Early, synsedimentary dolomite cements are extensively developed, particularly over local palaeohighs.Challenges to hydrocarbon production include: (1) the vertical variability of reservoir properties; (2) the presence of many, laterally extensive vertical flow barriers; and (3) variable connectivity between fault-bounded compartments. This complexity was underestimated at the appraisal stage, the initial development plan calling for a relatively simple scheme of pressure support to the critical reservoir fluid via injection into the gas cap and the water leg. This decision was supported by the identification of a single hydrocarbon column (>400 m thick) over the entire structure which suggested a lack of vertical compartmentalization.Recent studies, which include a geological synthesis of more than forty wells and a dynamic data synthesis to determine the production mechanism and identify key heterogeneities, show a lack of pressure support to the oil leg from both water and gas injector wells. Today, integration of static and dynamic data together with an improved geological understanding of the stratigraphic control on vertical flow barriers and mapping of high permeability sandstone layers is the key to identifying unswept zones of the field and future infill drilling targets.

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The Optimum Injection Rate for Wormhole Propagation: Myth or Reality?

2009

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The success in matrix acidizing carbonate-formations depends entirely on the ability to create long wormholes that bypass the damaged near-wellbore region. Recently, much work has been performed to understand the wormhole propagation. This has led to various criteria and models to optimize matrix-acid treatment design for volumes, fluids, and flow rates. In many publications, the existence of an optimum flow rate for wormhole propagation has been described and efforts have been made to design treatments around these criteria. This paper will discuss several of the criteria and models that can be used to determine the optimum flow rate in both radial and linear flow and will introduce a new and simple criterion for both radial and linear flow that is based on diffusivity, reactivity, porosity, and surface area. Most of the criteria have been developed based on laboratory results or simulations in controlled and idealized environments. Under field conditions, many additional factors must be considered. Because of certain parameters such as significant heterogeneities in the formation, uncertainties in formation data, uncertainties in fluid placement, and operational constraints, the environment is neither controlled nor ideal. This paper will discuss the impact of these field conditions on the criteria and models to determine the optimum injection rate and verify their validity under field conditions. Examples and a case history are included to emphasize the need for consideration of these parameters. In addition, constraints are given to the validity of the criteria for the optimum flow rate under field conditions. This paper will further discuss the best practices and recommendations for cases where these constraints cannot be easily met or in cases where the data is uncertain. Introduction The major goal of acid stimulation under matrix conditions into carbonate formation is to create conductive flow channels known as wormholes that bypass the damage in the formation. These flow channels make the connection between the hydrocarbon-containing formation and allow the hydrocarbons to flow into the wellbore when the well is put back on production. The wormholes are formed when the matrix of the porous and permeable rock is dissolved by reactive fluids, such as hydrochloric acid (HCl). In contrast, during sandstone stimulation, only the damage inside the pores can be removed and typically wormholes are not formed. The practical implications are that matrix stimulation in sandstones can only restore a well's natural productivity (skin = 0) by removing damage if the damage skin is acid-soluble; whereas, matrix stimulation in carbonates can truly stimulate the productivity in a well because the highly conductive wormholes can penetrate beyond the damaged zone and result in a negative skin. To obtain this negative skin, the treatment must be performed in such a way that deep penetrating wormholes are generated. Three important parameters that can be controlled need to be considered: fluid volume, injection flow rate, and fluid type. The fluid volume is an import consideration because more volume will generally lead to deeper fluid invasion and to potentially deeper wormhole penetration. The second important aspect is the injection flow rate. Under some conditions, typically when the injection flow rate is too low, wormholes are not formed. Instead, only some of the permeable rock is dissolved in the vicinity of the wellbore. At the other end of the spectrum, when the injection flow rate is extremely high, one consequence could include the development of very thick wormholes that do not penetrate deep enough. The results of these two flow extremes have been observed in core tests and led to the observation that there is an optimum flow rate. The third parameter that can be controlled is the fluid type. Some fluids will behave better for wormhole propagation under known reservoir conditions, such as temperature, mineralogy, and permeability. Variations in acid types, concentrations, additives such as gelling agents, and emulsions affect the wormhole propagation.

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Acid Stimulation of Extended Reach Wells: Lessons Learnt From N'Kossa Field

Cited by 5 publications

References 18 publications

An Innovative Methodology for Real-Time Control and Optimization of Dual Injection Increases Efficiency of Carbonate Stimulation in Horizontal Openhole Water Injectors

An Innovative Methodology for Real-Time Control and Optimization of Dual Injection Increases Efficiency of Carbonate Stimulation in Horizontal Openhole Water Injectors

Integrated approach to geomodelling and dynamic simulation in a complex mixed siliciclastic–carbonate reservoir, N'Kossa field, Offshore Congo

The Optimum Injection Rate for Wormhole Propagation: Myth or Reality?

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