Dariusz Strapoc

L. Gerbaud², D. Strąpoć¹

¹SLB, Clamart, France ; ²MINES ParisTech, Fountainebleau & Pau, France

Drillbit metamorphism (DBM) can generate gaseous products, including hydrogen from the decomposition of drilling fluids: oil-based mud (OBM), water-based mud (WBM), and even pure water, due to intense interaction at the drillbit-rock interface. Here for the first time, we study this phenomenon by varying only one parameter at a time and observe properties of the generated gases in an indoor rig floor facility. We derive correlations of drillbit-energy and vibrations proxies with amounts of generated gases, depending on mud type, and including alkenes, alkanes, hydrogen, and carbon monoxide.

Drilling was performed using one of a kind controlled-environment indoor-rig-floor with closed mud-loop connected to a gas logging equipment. The set up simulates drilling conditions down to 5 km using 0.5-m long rock samples. Controllable drilling parameters are weight on bit (WOB), rpm, mud flow rate, rock type (sandstone, granite, and basalt), drillbit type (several types of PDC bits with different cutter designs and level of wear, and different drilling fluids (OBM, WBM, pure water). A mud degasser was connected to the mud flow loop and mud gas analysis while drilling: i) gas chromatography (GC) for gaseous alkanes and alkenes, ii) mass spectrometry (MS) for H₂, CO₂ and hydrocarbons, iii) electrochemical for H₂. Additional spot samples were taken for detailed laboratory analyses. Subsequently, gas and drilling data, including torque variability and vibrations (triaxial accelerometer) were correlated.

Progressive and stepwise gas generation increases were observed in response to modulated drilling parameters, e.g., stepwise increase of rpm or WOB. We performed over 30 runs modulating an individual drilling parameter and using different drill bits with different levels of wear and different drilling fluids. In-depth data analysis shows that inefficient drilling, e.g., using a worn drill bit is prone to high heat generation, especially when increasing rpm, while newer harder drill bits tend to generate increased gas via higher levels of vibrations. In OBM gas molecular and isotopic data proves systematic cracking of base oil and generation of gaseous products, H₂ as the main one followed by methane, carbon monoxide, ethene, ethane, propene, etc. Moreover, these gaseous products tend to maintain constant molecular and isotopic ratios in individual runs and enable DBM-correction of naturally encountered C₁-C₅ alkanes and H₂ in exploration wells drilled with OBM. Additionally, we studied different heat-proxies using available drilling parameters, e.g., power input (torque multiplied by rpm) or high frequency torque oscillations, and vibrations, that influence quantities of DBM-generated gases, even when drilling with pure water. In the latter case only elevated H₂ was observed with traces of C₁, only when polymer mud additives were added to water. It is the first to date proof that pure water can be decomposed to H₂ by the act of drilling. Specifically using PDC drill bits on granite or basalt, and more so when increasing WOB causing enhanced vibrations. That implies that the water molecules can be split into H₂ via the act of extreme friction, high frequency impacts, mineral crushing, resembling the natural process of cataclasis known for natural H₂ generation during rock crushing at the activated fault zones.

The proven existence of the DBM gas by-products can flag and de-risk crucial mud gas logs for critical subsequent sampling, testing and completions decisions during hydrocarbons and H­₂ exploration. These successful experiments pave the way for systematic investigation of the DBM process to a wide extent, as a cautionary tale about practices used during exploration for petroleum and natural hydrogen.

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