São Francisco Basin Tight Reservoir Play: Defining Benefits of Hammer Bit/Percussion Drilling, Onshore Brazil
Authors: Gerlero Martin , Soroldoni Thiago , Barreto Diogo 
Published: June 2014

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 in the central state of Minas Gerais, 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. These gas rich formations present an important mixture of medium to hard lithologies that are at the same time highly dipped and faulted. This represents a great challenge for the drilling engineers and service companies while trying to maintain the verticality of the well and drilling at a competitive rate of penetration (ROP).

During the last two years drilling activities in the basin have intensified as the need for an abundant source of energy becomes more evident. This necessity is directly proportional to Brazil's economical surge in the last decade. The invigorated economy is consuming more goods and energy and the existing traditional ways of powering the economy might no longer be sufficient. The answer appears to be natural gas, already proven to be abundant in the area but held captive in super tight formations.

When oil companies were approached, they set for their main objective to drill vertical wells into the gas producing formations in the most efficient manner as possible. In Brazil's land drilling activities, economics are crucial due to the characteristics of the oil and gas companies and the high number of wells needed to produce at commercial levels. Since then, the operating companies have experimented with several drilling technologies attempting to extract the hydrocarbons at an economic level. Field trials include impregnated bits on turbines, hybrid bits, fluid/electromagnetic hammers and high speed motors (PDM) and rotary steerable tools (RSS). All BHAs met the goal with limited success and two major issues, lack of ROP and hole deviation problems. To solve the daunting 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. This way more wells could be drilled cheaper and much faster.

As a result of this under balanced drilling approach, two wells were successfully drilled with percussion hammers which enabled the operator to maintain verticality from top to bottom with less than 5° deviation and at a much higher rate of penetration. This increased performance, in addition to the better control of verticality allowed for a reduction of total drilling time and translated to reduction in total well cost. 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 with conventional methods, the average time was no less than 50 days, depending on the area of the basin and targeted TD.