No abstract

All tight gas project appraisals are inherently dependent upon the ability to execute effective fracturing treatments; that allow the determination of sustainable production rates; and hence potential project economics. Unlike the North American tight gas business, where thousands of wellbores are drilled each year and operational infrastructure and service quality is proven and in place; the International gas market challenges are significantly different. Often, the key risk with international appraisal; is not the presence of the hydrocarbons themselves, but more likely a competent and efficient execution of the fracturing process; that is essential in order to maximize gas inflow performance required for optimum well productivity. The Khazzan-Makarem structures have substantial known hydrocarbon reserves; which to date have resisted an economic development. The unique reservoir challenges of these formations have been shown to require specialised operational, execution and technical expertise in order to be successfully appraised. The approach taken by the operator was to design the appraisal project with particular emphasis on the application of massive hydraulic fracturing treatments; as without successful stimulation, it was appreciated that the ability to evaluate well productivity and hence the development potential would remain elusive. The result of close cooperation between the operator and the service providers, included planning the stimulation strategy, the number/sequence of wells, well types, fracture design, development of a matrix and selection criteria for technology deployment as well as operational issues associated with individual treatment execution. A locally tailored and carefully planned workflow was developed, that addressed the fracturing design process, the impact on the service delivery and the overall performance of this extensive project. Information presented within this paper, will provide a complete understanding of the specific factors involved in tight gas stimulation (particularly in remote areas). Furthermore, it will demonstrate a methodical approach to the process of planning, execution, QA/QC and post treatment assessment.

One often overlooked aspect of hydraulic fracturing is that the majority of reasonable permeability reservoirs (0.1 mD and above), on the North American continent, were in fact originally completed with straightforward hydraulically fractured vertical wells. However, as the average permeability of formations being developed deteriorated, this triggered a transition to multi-stage fractured horizontal wells and not unreasonably the fracture design and techniques that were developed to move from stage to stage were designed to be fit for purpose in these much lower permeability environments. These approaches, while suitable for lower permeability and unconventional formations, are not necessarily appropriate for higher permeabilities and conventional reservoirs. The Khazzan development in the Sultanate of Oman Block 61 includes a multi-layered, reasonable permeability, gas reservoir, which may be categorized as a tight gas reservoir. In such tight gas developments, fractured vertical wells have historically been the preferred completion design, due to favourable economics. Following an extensive appraisal programme, the development of the Barik reservoir in Block 61 was approved in February 2014, and while successful appraisal had taken place with fractured vertical wells, not unexpectedly multi-stage fractured horizontal wells were subsequently proposed as an additional incremental improvement option for development. In order to successfully achieve this, a number of standard operational practices and assumptions associated with North American unconventional horizontals needed to be challenged, adapted and in some cases stopped. The technical journey to deliver an effective multistage well design included an assessment of the impact of assumptions and considerations that drive unconventional practice, eventually leading to the road map to success that was developed. The learning includes three key pilot horizontal wells and clearly demonstrates incremental progression that was achieved, including technical obstacles faced, engagement with a complex stress-regime and how unconventional technology has to be adapted to be fit for purpose for the formation at hand. Not a static solution, the Khazzan development continues in the initial phase with fractured vertical wells achieving a rapid learning-curve and multi-stage fractured horizontal wells being optimized further. The experiences and outcomes from the suite of wells in this project demonstrated that multistage fracturing of horizontal wells requires careful consideration, particularly in the selection of technologies and their application. The approach adopted in this project has led to developing the field with a healthy suite of competing completion techniques that offer best-fit solutions under different scenarios, and this set of complementary options will ensure that the development economics are maximised.

In the majority of fractured oil and gas wells, conventional perforating is the typical approach of choice to provide the primary connectivity of fractures to the wellbore, and in horizontal wells the very discrete nature of this connection assumes a significantly higher importance. In multi-fractured horizontal wells, this connection drives the ability to efficiently place the fracture treatments during pumping and the efficiency with which the fracture can subsequently be produced. Consequently, selection of the most appropriate connection technique can be absolutely key to many aspects of a successful implementation of a fracturing campaign. The use of shaped-charge perforating is quite commonplace and predominantly considered as best practice for the majority of scenarios, in order to establish fracture/wellbore connectivity. However, there are certain situations where such approaches may not provide an efficient solution. This is particularly true in those horizontal wells drilled and completed in complex stress regimes, also in reasonable permeability reservoirs, that have multiphase flow potential or with just a few transverse fractures that are expected to produce at moderate to high production rates from each frac. In these particular cases, a complex connection resulting from perforating can often be detrimental to fracture width creation, making proppant placement challenging and reducing effective fracture conductivity. Additionally, convergent and multi-phase flow behaviour can create extremely high pressure drops in the near wellbore area subsequently impeding the productivity. While open-hole completions can be one of the methods to deal with this situation, by effectively eliminating the "problem" at source, this is typically delivered at the expense of loss of control on the point of fracture and also with a statistical isolation failure rate. When this is implemented in multistage/multi-cluster frac environments (effectively hundreds of fracs) such statistical failure is an acceptable risk. However, when a single-well frac count is just 3, 4 or 5 per well, any statistical failure can be materially impactful on the well productivity. In those cases when open-hole is not an attractive approach then cased-cemented is preferred, and the application of abrasive jetting can provide an effective alternative to the use of shaped-charges. This paper will fully describe a suite of tests performed with different shaped-charges as well as abrasive jetting perforators, static holes and dynamic slotting for the multi-fractured horizontal wells in the Khazzan tight-gas condensate field in the Sultanate of Oman. The paper will also include a comprehensive review of multiple injection tests that were performed in both Khazzan vertical and horizontal wells (Al Shueili et al., 2016), through both shaped-charge and abrasive jetted connections. This review will offer observation on maximising the effectiveness of the pre-frac wellbore connection technique in challenging environments.

This paper will cover the design and installation of BP’s first 15Kpsi Open Hole (OH) completion in the Sultanate of Oman in March 2016 and also the initial execution results of the stimulation treatments in July/August of the same year. This change to the current well design and execution strategy has become necessary due to variable cased hole horizontal well results and greater understanding of the challenges of delivering efficient and sustained gas production from the higher fracture gradient areas of the Barik reservoir. The Khazzan (Barik) development in the Sultanate of Oman operated by BP is a tight gas project, requiring hydraulic fracturing of tight gas resources. Tight gas production from the deep hot reservoirs in the Sultanate of Oman has historically concentrated on cased hole completions stimulated with large hydraulic fractures. The original Basis of Design for Khazzan for Full Field Development consisted of horizontal wells with multiple hydraulic fracturing stages performed within a Cased & Perforated 4-1/2″ liner design. Challenges encountered with the CH approach have included the following: Tight pressure deployment/pumping window within the existing completion design, complicated by a wide variation in areal and vertical stress regime.Variability in the injection response, proppant placement and particularly the quality of the fracture/wellbore connection that would and has been achieved.Lack of predictability regarding post-frac production rate, due to a variation in fracture placement achieved due to above two reasons. In 2014 a decision was made to introduce some flexibility in evaluating suitable fracturing designs incorporating a number of Lower Completion (LC) styles, one of which was a horizontal open-hole completion. Optimizing stimulation performance by evaluating ball dropped activated systems and over-displacement was seen to have significant potential. A multi-disciplinary approach involving drilling, completion, stimulation, intervention and subsurface was performed to ensure Project value was maximized and the objectives delivered. This paper will cover how the pressure rating of the OH completion was designed to 15 kpsi, in excess of the existing CH pressure rating. This includes screening and evaluation of the available open-hole LC system design and operational characteristics suitable to deliver propped gel fractures in 6″ OH for 1,000 m horizontal wells. Screening criteria included; system/equipment technology status, associated drilling requirements and design for "5-7/8″ hole" cost and duration versus the existing 8-3/8″ hole configuration and an ability to meet the well stimulation Statement of Requirements (e.g. fracture placement, zonal isolation). The final system design will be detailed including wellbore orientation and trajectory, hole and casing sizes, zonal access and isolation method(s) and hydraulic fracture parameters, including fracture spacing, geometry and treatment design. Operational results will be presented for well construction and stimulation phases. Well construction results will include drilling performance comparison, wellbore preparation for the completion installation, drill-in/completion fluid requirements and packer spacing/zone selection criteria. Stimulation execution results will include evaluation of execution versus design, comparison of stimulation results for ball drop stage results versus plug and perf results. Assessment of Radioactive (RA) tracer results and evidence for OH packer integrity will also be presented. Conclusions will include an initial comparison of the execution of the drilling, completion and stimulation phases and lessons learned on the success of the design versus the original objectives.