New tools and techniques have rejuvenated the directional drilling business in recent years and have quickly taken conventional directional assemblies to near-upper drilling limits. Rotary steering tools were, and still are, being developed to eliminate some of the drilling limitations encountered with conventional assemblies. However, good alternatives are available to help drill well paths of increasing complexity and reach.
Beyond older technologies, such as thrusters and percussion drills, a new line of tools is being developed to give the drillstring "extra motion." These tools provide directional drillers with the ability to overcome increasing axial and longitudinal friction forces. In order to extend the capabilities of conventional steering and new rotary steering technologies, Andergauge Drilling Systems has developed the Fluid Hammer Drilling System (FHDS). The FHDS expands the operating envelope for conventional directional drilling techniques with a benign effect on tubular and drilling tool mechanical performance.
Directional drilling
Recently, two wells were drilled in the Brent formation of the Northern North Sea using the FHDS within conventional steerable assemblies.
The diverse geologic makeup of the Brent formation and its depleted state, results in directional steering difficulties. Sliding below 2,400 meters true vertical depth (TVD) is almost impossible with PDC bits, and bit selection is reduced to roller-cone rock bits in directional steering situations.
The primary objective of running the FHDS was to establish if a PDC bit could be conventionally steered across the complete Brent sequence. With numerous platforms re-drilling complex 8 1/2-in. holes through this formation, the economics of increased ROP and the elimination of possible bit runs were of key importance.
(1) The FHDS was run twice on the first well. The first run only drilled 178 meters of new hole before high pressure pulses started to corrupt the measurement-while-drilling (MWD) signal. However, as this short interval showed that slide ROPs of 40 meters/hour could be achieved, the operator showed no hesitation in picking up a second FHDS. The second PDC bit run drilled 836 meters into the depleted Brent sandstone in 29.7 hours, with no reductions in slide ROP, before similar MWD signal deterioration terminated the run. A steerable turbine assembly was then selected for the next bit run with an accidental sidetrack the result. The subsequent inability to slide a PDC bit and steerable motor over the identical geologic interval during the later sidetrack highlighted the significant potential savings of using the FHDS. A second target well was then selected and the FHDS was modified to overcome the premature bearing degradation that was causing the high pressure pulses and MWD interference.
(2) In the second well, the FHDS drilled 573 meters of the Brent formation in a single 48-hour PDC bit run. This included 258 meters of oriented slide drilling, at an average ROP of 12 meters/ hour. Drilling weight on the PDC bit was maintained at 6,000 lb during the bit run. After some fine-tuning of hydraulic parameters, no problems were experienced in decoding the MWD signal, and the FHDS ran for more than 70 circulating hours without a problem. Three subsequent steerable motor assemblies made a total of 191 meters in 58 hours. The first, with a rock bit, made 127 meters. Slide-drilling ROP was an unfortunate 1.9 meters/hour. In an attempt to increase ROP, the next run used a PDC bit, resulting in a total inability to slide and only 2 meters of progress. The third run used a rock bit to orient and steer the well back to planned trajectory. This required 68% of total drilling time in a steering mode and a resulting ROP of 2.4 meters/hour. An Ander-gauge adjustable stabilizer, in a near bit position, with a PDC bit, completed the 519 meter section in 37 hours.
Not traditional concept
Older, conventional percussion drilling concepts use the hammer-anvil principle to effectively "spud" the bit by adding more vertical impact force to fracture the rock and increase penetration rate. This increased impact force severely damaged polycrystalline diamond compact (PDC) bits, as these bits do not sustain vertical weight loads effectively.
The FHDS technology works on a fluid hammer principle to effectively add longitudinal energy to the drillstring. The action is similar to a drilling jar, but with no hammer-anvil effect, and substantial lower stroke power. Hydraulic pulses generated by the FHDS are translated into mechanical energy in the shock sub, which is transferred to the drillstring. The idea is not to create impact forces on the bit, but |to create added longitudinal energy in the drillstring and bottomhole assembly (BHA) for friction reduction.
Suggested tool configuration within the BHA consists of the FHDS and a shock tool further up in the bottomhole assembly. A rotating valve, driven by a positive displacement Moineau-type motor within the tool, causes a cyclical restriction to the mud flow, producing a series of pressure pulses.
These internal pressure pulses act on the seal area of the spring-loaded shock tool mandrel, causing it to extend and retract, thus producing a slight oscillation of the BHA and cyclical variation in weight on bit (WOB). The tool produces large internal pressure pulses, but only shows an actual pressure drop of approximately 500 psi on the driller's console gauges.
The longitudinal forces generated on the drillstring are not enough to lift the bit off bottom, eliminating any "spudding" action on the bit. With no impact forces produced, the tool does not require special bits and has been successfully run with both roller-cone and PDC bits.
Growing pumping life
Post-inspection of FHDS tools in early runs revealed significant bearing damage in the bearing assembly. More recent case studies have shown that the expected life of the tool is now 100 pumping hours at 540 gpm, and 13 ppg mud weight. The target for pumping life is 200 hours within the next three months.
Operational parameters of the tool generate little interference with other service tools. Damage to conventional steering motors has been minimal to none. Bit wear appears to be normal. Roller-cone rock bit runs over 50 hours in length have been logged with no bearing damage.
Pressure pulse frequency of the FHDS is flow dependent, dictated by the motor section driving the rotating valve. Increasing the flow rate increases the frequency, and there is minimal MWD signal interference or damage from the induced pressure pulses.
New impact tool
The FHDS is intended to provide longitudinal "excitement" of the BHA to significantly increase the operating window in which conventional steerable assemblies can be used with PDC bits. Andergauge is developing a reciprocating mass tool that will also provide an impact force at the cutting interface, to significantly increase the rate at which hard rock can be drilled.
Such technology exists within the mining industry, but importing the tools into a drilling fluid environment has brought longevity problems associated with internal erosion related to sand and solids. Andergauge hopes to overcome these problems by combining the proven mechanics of the FHDS with a modular extension housing the hammer.
The reciprocating mass variation will undergo field trials at the end of this year. With many operators looking at marginal fields in harder rock, the ability to reliably increase ROP will ensure keen interest in the further development of the FHDS.
