Abstract
Raageshwari Deep Gas (RDG) field is a major gas field in the Barmer Basin of Rajasthan, India which comprises of a tight gas-condensate reservoir within the underlying thick Volcanic Complex. The Volcanic Complex comprises two major units – upper Prithvi Member (Basalt) and lower Agni Member (Felsics interbedded with older basalt). The production zone is drilled in 6" and has historically seen high level of shock & vibrations (S&V) and stick-slip (S&S) leading to multiple downhole tool failures and poor rate of penetration (ROP).
Individual changes in Bit and bottom hole drilling assembly (BHA) design were not able to give satisfactory results and hence an integrated approach in terms of in-depth formation analysis, downhole vibration monitoring, correct predictive modelling, bit and BHA design was required.
A proprietary formation analysis software was used to map the entire RDG field to understand the variation in terms of formation compactness, abrasiveness and impact (Figure 1,2,3 & 4). The resulting comprehensive field map thus enabled us to accurately identify wells that would be drilling through more of problematic Felsics and where higher S&V and S&S should be expected.
To better understand the vibrations at the point of creation, i.e., bit, a downhole vibration recording tool was used to record vibration & stick-slip data at a frequency of 1024Hz. This tool picked up indication of a unique type of vibration occurring downhole known as High Frequency Torsional Oscillation (HFTO), that was quite detrimental to the health of bit and downhole tools. A proprietary predictive modelling software was used to optimize the bit-BHA combination to give least amount of S&V and S&S.
Data from the downhole vibration recording tool, formation mapping software and offset bit designs was used to design a new bit with ridged diamond element cutters and conical diamond element cutters to drill through the highly compressive and hard basalt. The predictive modelling software identified a motorized Rotary steerable assembly (RSS) to give the best drilling dynamics with the newly designed bit. The software predicted much lower S&V and S&S with higher downhole RPM which was possible with the help of motorized RSS. Implementing the above recommendations from the various teams involved in the project, drilling dynamics was vastly improved and ROP improvement of about 45% was seen in the field. This combination was also able to drill the longest section of Felsics (826m) with unconfined compressive strengths as high as 50,000 psi in a single run with excellent dull condition of 0-1-CT-TD
This paper will discuss in detail the engineering analyses done for improving drilling dynamics in field along with how HFTO was identified in field and what steps were taken to mitigate it.
