Fracture Treatment Optimization for Horizontal Well Completion M.Y. Soliman, Reinhard Pongratz, Halliburton; Martin Rylance, TNK-BP; and Dean Prather, Halliburton
Copyright 2006, Society of Petroleum Engineers parameters influencing the completion of a well. Fluid flow and This paper was prepared for presentation at the 2006 SPE Russian Oil and Gas Technical geomechanical aspects of fracturing a well cannot be ignored Conference and Exhibition held in Moscow, Russia, 3–6 October 2006. when multiple fractures are created. This is especially true in This paper was selected for presentation by an SPE Program Committee following review of case of fracturing horizontal wells. information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to Although unstimulated horizontal wells have been very correction by the author(s). The material, as presented, does not necessarily reflect any position successful in naturally fractured reservoirs and in reservoirs with of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society of Petroleum gas- or water-coning problems, there are many situations where Engineers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineers is prohibited. fracturing a horizontal well to improve production capability is a Permission to reproduce in print is restricted to an abstract of not more than viable or necessary option. The orientation of a hydraulic 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box fracture, with respect to the wellbore, is directly related to the 833836, Richardson, TX 75083-3836 U.S.A., fax 01-972-952-9435. wellbore azimuth with respect to the in-situ stress field. Therefore, the possibility of fracturing a horizontal well must be Abstract considered before the well is drilled. The appropriate Fracturing has become a viable and important option for contingency plans should be made to anticipate the possibility of completing horizontal wells. There are many fracturing low productivity from an unstimulated well. processes and methods to consider for placement fractures. It should also be remembered that fracturing a horizontal Optimization of the completion process including the number well may dictate which direction the well should be drilled and and size of fractures is still a challenge. how it should be completed. Fracturing a horizontal well does Although fundamentally similar to fracturing vertical wells, not necessarily mean that the well has to be cased and cemented. horizontal well fracturing has unique aspects that require special There are many cases of fracturing horizontal wells in openhole attention to ensure successful treatment. Differences exist or uncemented liners. The field example within this paper between horizontal and vertical wells in the areas of rock discusses one of these cases. Fracturing a horizontal well may be mechanics, reservoir engineering, and operations. These aspects considered when one of the following situations is apparent. affect the optimization process for successful placement of ， Restricted vertical flow caused by low vertical treatments and optimum asset performance. permeability or lamination. In this paper we discuss the various factors crucial to ， Low formation productivity because of low formation successful completion of a fractured horizontal well. We discuss permeability. these factors in relation to both longitudinal and transverse
， Low-stress contrast between the pay zone and the fracture applications. Success factors include the optimum
surrounding layers. In this case, a large fracturing perforation process, overcoming fluid flow convergence towards
treatment of a vertical well would not be an acceptable the wellbore in case of a transverse fracture, and the fluid flow
option because the fracture would grow in height as well as and stress interference between multiple fractures.
length. The paper presents a field case and laboratory and numerical experimentations illustrating the impact of the various factors on
The effects of lamination within a reservoir body are not the completion of the horizontal wells and the optimization of
generally considered, or their importance underestimated when the fracturing process.
designing horizontal well completions. The presence of laminations is considered to be one of the principal reasons Introduction
behind poor performance of a number of horizontal wells. A Fracturing is no longer restricted to vertical wells drilled in hard
significant percentage of moderate- and low-permeability formations with very-low permeability. Higher permeability,
formations are laminated with low-permeability streaks that softer formations as wells as horizontal wells are now routinely
result in a low effective vertical permeability. Even a very thin fractured. This has led to the importance of examining all
barrier that may be too thin to readily detect can form a barrier reservoir aspects to reach a better understanding of efficient
that essentially prevents hydraulic communication between the fracture design and eventually the optimization of the well
horizontal well and the formation beyond this barrier. In these completion. This should include the theoretical and operational
cases the creation of a number of vertical transverse or
2 SPE 102616
longitudinal fractures would allow the effective drainage of the transverse fracture to be less effective than a fracture reservoir through these fractures. intersecting a vertical well. The two flow regimes are identical Although fundamentally similar to fracturing vertical wells, for infinite conductivity fractures. This indicates that transverse fracturing horizontal wells has unique aspects that require fractures may not be recommended for higher-permeability special attention if the most successful treatment is to be formations unless severe formation damage is expected around secured. Differences between horizontal and vertical wells exist the wellbore and the fracture is merely designed as a means to in the areas of fluid flow, rock mechanics, perforation strategy, connect the wellbore to the undamaged formation. The problem and operational procedures. It has already been well established with this convergence of fluid is increased because of the non- that the two extreme fracture orientation cases are transverse and Darcy effects in the fracture that would be expected in the case longitudinal fractures. The various aspects of fracturing of higher production rates.
horizontal wells for both transverse and longitudinal fractures Because high pressure drop is expected because of the fluid- are examined in the next few sections as pertaining to flow convergence that occurs around the entry to the transverse optimization-fractured horizontal wells. fracture, tailing in the pumping stages of a hydraulic fracturing
operation with a high conductivity “tail-in” proppant would be
recommended. Hydrajetting the wellbore before fracturing and Fluid Flow Aspects of Fractured Horizontal Wells
The most significant advantage of this approach is the creation packing the fracture during the later stages of fracturing with a of multiple parallel fractures resulting in an acceleration of larger and/or stronger proppant is highly recommended. hydrocarbon production. The creation of multiple fractures Because the radial-linear solution is valid only at early time, allows an efficient drainage of the reservoir with fairly small several authors10-13 have expanded their investigations to study
the flow regimes in the reservoir at a later time. Roberts, et al10 fractures. To reach the same effect by drilling a vertical well
would require a significantly longer fracture that would also presented a description of the expected flow regimes linear-
radial, formation-linear, compound linear, and finally, pseudo- have a greater height. This would increase the chance of
fracturing through a gas cap or water aquifer. The inclusion of radial flow regimes. These flow regimes are illustrated in Fig. 1.
unwanted gas or water production, as well as fracture A transition period is expected between the various flow
regimes. Some of these flow regimes may not be apparent, communication with a fault boundary, would negatively affect
production. depending on reservoir extent, continuity, and geometry. The increased productivity caused by the presence of
transverse fractures has been studied by many authors.1-16 An early work in this area was presented by Karcher, et al.1 Giger2 used a numerical model to study the steady-state production increase from a horizontal well with multiple, infinite conductivity vertical fractures. Work by Giger indicated that as the number of fractures increases, and as they get longer, the production through the fractures will eclipse the production from