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To maintain high production rates each well’s potential is to be exploited to the full, for example through well interventions such as hydraulic fracturing, acid treatments, etc. In 2009, specialists in the Well Intervention Planning Dept. of Samotlorneftegaz together with their colleagues from TNK-BP Corporate Center proposed an unprecedented integrated approach to fracturing in horizontal wells with an uncemented perforated liner in the horizontal section that requires no additional perforation. In recognition of the method’s efficiency and great potential the project team was awarded the TNK-BP Technology Award in 2009.
Samotlorneftegaz first applied hydraulic fracturing in horizontal wells in March 2007. Since then, the method has been most widely used to develop high-permeability BV10(1-2) reservoirs and the low-permeability formations of AV and YuV groups. In 2009, the frac program was extended to include horizontal wells in BV8(0) reservoir, which has a thin net pay (2.5 m to 4 m), is relatively unsegmented, and has a permeability of 30 mD. The development of this formation is additionally complicated by an underlying aquifer.
The need for inflow stimulation techniques in the wells of BV8(0) reservoir was driven by the substantial drop in productivity of horizontal wells drilled in 2005-2008: by late 2008 many of them were operating on choke or intermittently, with an average water-free flow rate of 10.6 tpd. Possible reasons for this drop in production are:
» Clogging of the bottomhole zone during drilling or completion
» Incorrectly selected or insufficient treatment to remove mud cake during completion
» Loss of permeability in the bottomhole zone during well operation due to movement of fine particles, changes in liquid phase or rock deformation
Repeated acid treatments of the bottomhole zone and plastic perforations using a slotted hydromechanical perforator failed to produce the expected result. Some wells had become sub-commercial or were shut down due the absence of inflow. Frac efficiency in a number of horizontal wells in BV8(0) reservoir also proved rather low due to fracturing of the underlying aquifer. As Samotlorneftegaz specialists were analyzing the reasons for the failures, they discovered that the geometry of the well’s horizontal section had a substantial effect on the well’s flow rate after frac; no effect was established of the wellbore spatial orientation on the well productivity index (Fig.1).
A process for selecting and evaluating candidate wells for frac therefore had to be devised to avoid future systemic errors and mitigate technological and geological risks. Samotlorneftegaz specialists were assisted in this task by their colleagues from SNGDU-2, TNK-Nizhnevartovsk, Upstream’s Wellwork Dept. and Technology Stream.
Wells with a Twist
With proper understanding of the factors affecting a well’s flow rate after frac it became possible to approach the task of inflow stimulation in horizontal wells in a new way. The basic idea was to alter the geometry of the horizontal section of the wellbore to create the best conditions for subsequent frac in the formations of AV, BV and YuV groups.
A typical horizontal section of a well is drilled with a slanted or sinusoidal wellbore geometry (Fig.2); the length of the horizontal section is 200 m to 300 m.
The team used geomechanical modeling tools to devise a brand new geometry for the well’s horizontal section that will boost frac efficiency. The well makes a steep entry into the formation before rising gently in such a way that the end of the wellbore is higher than the entry point or the point at which the slanted part of the horizontal wellbore is cut off by the unperforated liner pipes (Fig.3). This uprising well profile can be drilled irrespective of the direction of the horizontal wellbore in relation to the minimum horizontal stress axis.
When wells with a vertical termination are fractured, the rock pressure at the end of the bore is lower than in other sections and a fracture forms at the end of the horizontal section. With such a fracture the drainage zone within the formation can be expanded, thus increasing the well’s flow rate by several times.
The bottomhole pressure is distributed evenly along the horizontal bore and therefore the flow enters the well first in the horizontal section, and only later from the newly created fracture at the end.
New Geometry Brings Excellent Results
Altering the candidate well selection criteria in favor of horizontal wells with an uprising profile has resulted in a substantial increase in the productivity index and oil production after frac.
In BV8(0) reservoir 15 fracs have been performed in wells with an average initial flow rate of 10.7 tpd. After frac, the average flow rate increased by almost four times and reached 41.1 tpd (Fig.4). Moreover, following recommendations from the Well Intervention Planning Section of Samotlorneftegaz, in 2009 horizontal wells penetrating the formations of AV, BV and YuV groups were drilled with a gently stanting wellbore and an uprising termination. After drilling or sidetracking, fracs were carried out in 15 wells, producing an average incremental flow rate of 45.8 tpd. Over 10 months of 2009, the total incremental oil production from the 30 wells was 103,300 t.
Besides boosting flow rates and producing incremental oil the change in wellbore geometry has substantially cut the cost of drilling and sidetracking. First, owing to the half-length of the fracture the drainage area within the formation increases to 100 m after frac; therefore a horizontal section of 100 m to 150 m is sufficient in a new-style well, compared to 250 m to 300 m in a ‘conventional’ well. With the shorter wellbore the drilling time and, therefore, costs (mainly telemetry) are reduced. There is also an opportunity to reduce the number of wells: in place of two directional wells with subsequent fracs it is now sufficient to drill one horizontal well with an uprising profile and frac it.
As a result, drilling wells with an uprising geometry can cut the cost of producing one barrel of oil by up to 15 percent.
Future of the Technology
The project team estimate that this new frac approach in horizontal wells can ‘revive’ 51 wells in Samotlorneftegaz alone. The results of the frac efficiency analysis were considered when planning the subsidiary’s operations in 2009-2010:
» Horizontal wells and sidetracks with a horizontal section penetrating the formations of AV, BV8(0), BV10 and YuV groups will be drilled with an uprising wellbore geometry and subsequently fractured (46 wells)
» A screen-free filter with 16 m to 18 mm openings specially designed for frac will be used to case the horizontal section of the new wells; with no need for perforation before frac the costs will be further reduced
» Less expensive muds will be used when drilling horizontal wells and sidetracking (permissible where frac is to follow) to further reduce drilling costs
» Two fracs are planned in each horizontal well – at the start and at the end of the wellbore, thereby boosting productivity and maximizing oil recovery
The method proposed by Samotlorneftegaz to boost productivity in horizontal wells could also be used in other Company subsidiaries operating horizontal wells with low productivity indexes. The project team recommend introducing the new technique step-by-step,with pilot tests involving up to 10 wells to take account of the specific features of the different regions and fields where the Company operates. Using the experience gained in the Samotlorneftegaz project other TNK-BP subsidiaries can plan drilling of horizontal wells and sidetracking with horizontal sections in all formations with subsequent frac.
The project team are confident that for the Company as a whole this approach opens up new opportunities to develop hard-to-recover reserves and to optimize and intensify development of both low-permeability and high-permeability reservoirs.
For their part, the project designers plan to build further on what they have achieved so far. The search for new methods of enhancing oil recovery will continue; the share of tight oil reserves increases year by year, which gives permanent focus to the search for methods to improve well efficiency.
Published thanks to TNK-BP and Innovator Magazine