Technical Paper: São Francisco Basin Tight Reservoir Play: Defining Benefits of Hammer Bit/Percussion Drilling, Onshore Brazil

Society: SPE
Paper Number: 167926
Presentation Date: 2014
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Drilling activity in Brazil has been focused primarily on developing offshore oil reserves in the high-profile pre/post salt plays. However, test wells in the Sao Francisco basin have identified a vast opportunity for onshore natural gas production from unconventional tight sandstone and carbonate formations. The prospect is attractive because of its relatively shallow depth that varies from 2000 to 4000 meters and its close proximity to high population density. 

But constructing the 12 1/4-in and 8 1/2-in vertical hole sections through the highly dipping formations and complex fracture zones has presented a number of distinct challenges when drilling with a conventional motor/PDC bit. The high dip angles which typically fluctuate between 15-40° but can reach 60° frequently generate wellbore deviation up to 18-20 degrees. Exacerbating application difficulties, the overburden contains highly interbedded shale, hard limestone and abrasive sands with high UCS. The transition drilling through these difficult lithologies creates severe bit dulling, low ROP and catastrophic vibration levels resulting in frequent reamer/MWD and drive system failures. The combination of issues forces the operator to trip repeatedly for tool change-out driving up development costs in the economically sensitive play. 

The operator has experimented with several drilling technologies attempting to extract the hydrocarbons at an economic level. Field trials include impregnated on turbine, hybrid bits, fluid/electromagnetic hammers, high speed PDM/RSS. All BHAs met with limited success. To solve the challenges, the service company recommended drilling with nitrogen/air and hammer bits to increase ROP and maintain hole verticality to reduce/eliminate expensive correction runs. The suggested percussion BHA was run in the 12 1/4-in hole section through an interbedded and faulted formation and drilled at an average ROP of 14m/hr and 9.5m/h respectively for each of the two test wells. This doubles and almost triples the average ROP of 5m/hr with the PDC/motor BHA in this same formation. In the 8 1/2-in section, in a similar formation as the 12 1/4-in hole section, the hammer bit/percussion BHA again provided increased ROP drilling at 12-14m/hr in both wells. This represents an increase of 200% compared to the average ROP of 4.5m/hr with a conventional PDC/motor BHA in the same formation. 

Additionally, the percussion hammer enabled the operator to maintain verticality from top to bottom with less than 5° deviation on the two wells. This increase in performance, in addition to the better control of verticality allowed for a reduction of total drilling time and translated to reductions in the total costs for the well. The time to drill the two wells with air took approximately 30 days (wells to 1500-2000m), whereas in the other group of wells in the basin, the average time was of no less than 50 days, depending on the area of the basin and TD.

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