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

Abstract

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.

Rotary - percussion drilling method - historical review and current possibilities of application

“…The main problem to drill in describing basin were interbeddedshales, hard limestones and hig dip angles which frequently generate wellbore deviation in short drilled distances (up to 20 o ). All of described problems generate extra costs, that is the reason to compare results of drilling when conventional and rotary-percussion drilling method are applied [3].…”

“…Rotary percussion drilling has widely been recognized for its potential to drill faster, in comparison to conventional rotary drilling method, where traditional roller-cone and PDC bits had been used. This ability becomes more evident when the drilling takes place in hard formations like granits, sandstones, dolomites and limestones [3].…”

“…This system is in use until now. Retaining system has essentially eliminated the shank separation problem common on hammer bit [3]; 1 the primary retention mechanism is a retaining ring at the top of the bit, 2 the retainer consists of a sleeve positioned between the shoulder of the driver sub and the hammer case, 3 the secondary catch mechanism is a rope thread machined on the retainer and the bit…”

“…In Brazil, in the São Francisco basin, where tight gas plays occurs, instead of air, were applied mixture of nitrogen and air in order to eliminate or decrease risk of ignition. [3].…”

“…In hammer job-2, BHA drilled 474 m with ROP of 13.5 m/h. Drilling with 8 1/2" diameter, BHA drilled 850 m in the hammer job-1 (with ROP of 13.8 m/h) and 630 m in the hammer job-2, with ROP of 12.2 m/h [3]. Figure 4 shows the ROPs achieved in 16 wells made by conventional drilling, and two wells drilled with hammer BHA.…”