A development programme offshore Western Australia required near horizontal 8.1/2" wellbores to be drilled through challenging formations. The hole section intersected sand-shale interlayers, referred to as the Tiger Sands, which is an abrasive formation with unconfined compressive strength (UCS)between 15,000psi and 20,000psi, with maximum UCS recorded up to 30,000psi. Underlying the Tiger Sands is the well cemented, abrasive Brewster sandstone with average UCS of 15,000psi and maximum UCS recorded up to 21,000psi, which has demonstrated a propensity to natural fractures. These challenges resulted in poor bit durability, low rate of penetration (ROP), difficult directional control,excessive shocks and downhole vibrations and significant downhole mud losses. The complexity of the drilling conditions required a systematic approach to be employed to optimise the poly-diamond crystalline (PDC) drill bit solution in order to improve durability and optimise ROP while successfully managing directional control, losses and ultimate drilling cost through the interval.
In addition to drill log analysis, after operations reviews (AORs) provided a comprehensive summary of operations from a rig perspective. These summaries were then used by the onshore engineering team to work in collaboration with the service sompany engineering team to develop a bit optimisation strategy. The first development well of the campaign was drilled with a standard 8 bladed 13mm PDC bit.
Natural fractures were penetrated and a total of ~4,500bbl of synthetic sased sud (SBM) was lost to the formation. The bit suffered impact damage and, therefore, had reduced durability. To solve thisproblem, a systematic approach was followed to gradually introduce improved design elements. Bit design progressed from cutter design, improved technology and ultimately, the introduction of new PDC cutter technology to record the fastest on bottom ROP for the drilling campaign of 10.32m/hr. Improved ROP and optimisation of the loss treatment strategy, allowed drilling operations to continue with minimal exposure to losses when present. Positive AOR feedback and continuous development from the service company resulted in further improvements to the PDC bit design with the introduction of ridged diamond elements and the latest generation conical elements to further improve steerability and ROP.
Continued developments resulted in New Design 4 being run in well 10. The entire interval was drilled in one run with excellent steerability and an overall 159% improvement in ROP equating to a reduction in drilling time by 5 days going from the original 813 PDC to the final New Design 4. In addition to ROP improvements, losses did not prove prohibitive to reaching TD due to drilling efficiency. Subsequent further improvements in ROP have been realized, with the entire interval drilled at 16m/hr.
Rogers, Clint (Smith Bits a Schlumberger Company) | Jangani, Reza (Smith Bits a Schlumberger Company) | Spedale, Angelo (Smith Bits a Schlumberger Company) | Sadawarte, Sagar (Smith Bits a Schlumberger Company)
The Mereenie development project is targeting oil and evaluating natural gas reservoirs in the lightly drilled Amadeus Basin. In 2012, an operating company started searching for methods to improve rate of penetration (ROP) drilling the 8¾? vertical hole section through the difficult Stairway and Pacoota sandstone formations. The lithology consists of very abrasive and hard siltstone/sandstone with UCS up to over 30,000 psi. The hole section starts at 500 m and generally requires 700 m of total wellbore to reach KOP at 1200 m. The section has historically been drilled with PDC and Roller Cone bits with mud as the circulating medium. Both types of BHAs produced unacceptably slow ROP and required multiple trips to reach TD. The operator required a new approach.
To accomplish the objective, the operator wanted to switch from mud to underbalanced drilling using an air percussion BHA equipped with a hammer bit. However, an analysis using a well records database showed that only short (10–30m) shallow surface intervals had been drilled in Australia with percussion air hammers mostly in mining applications in the 1980–90's.
To increase the chance for early success, the operator wanted to import the latest air hammer tools and drilling techniques from North America. The provider suggested taking lessons learned from the Northeast USA where air hammer drilling plays a major role in developing oil and gas reserves in the region. The two applications are similar with regards to formation characteristics and the drilling team concluded the provider's downhole tool technology, service culture and experience/expertise would be integral to project success. In Q4 2013 the provider drilled the fastest and deepest percussion air hammer run in Australia's Oil and Gas history at 24 m/hr, 700% faster than the previous ROP achieved with PDC or Roller Cone.