--> Challenges And Solutions Associated With Drilling Deviated Wells In Alternate Soft-Hard Formations -A Case Study From Onshore Kg Basin, India

AAPG Asia Pacific Region GTW, Pore Pressure & Geomechanics: From Exploration to Abandonment

Datapages, Inc.Print this page

Challenges And Solutions Associated With Drilling Deviated Wells In Alternate Soft-Hard Formations -A Case Study From Onshore Kg Basin, India

Abstract

The cost of well construction can significantly exceed budget if drilling operations are plagued by wellbore-instability problems and excessive time and resources are needed to free stuck pipe, regain circulation, or clean the hole efficiently. Particularly while drilling high-angle wells, wellbore-related issues such as hole cleaning always remains biggest challenge, especially with a kelly rig system because a backreaming operation cannot be performed if tight spots are encountered. In such cases, RPM, flow rate, and good mud rheology are of utmost importance, and these, in turn, govern the effectiveness of hole cleaning. These challenges, if not handled properly, lead to severe wellbore failure, shortening total depth (TD), sidetracking of the hole, premature cancellation of the drilling program, and several other related problems. In addition, severe hole enlargement and rugosity through the eventual reservoir sections results in poor logging conditions and uncertain reservoir evaluation. Such issues have challenged access and interpretation of the reservoirs to prove and produce reserves in the study region.

Exploration is being done in the Godavari clay petroleum system of Krishna-Godavari (KG) basin to add new reserves within this challenging drilling environment. In this quest, due to logistics, positional and geographical complications, deviated “L” and “S” shaped wells are required from a land site to offshore. Drilling these deviated exploratory wells in the young Godavari clay formation proved delinquent, causing three lost in hole incidents (LIH) in a single well. Study aimed to investigate the root cause of these LIH incidents and other drilling-related issues like stuck pipe, tight spots etc. This study provides insight through integration of geomechanics with drilling and mud designing parameters to assist in decision making for future well planning.

Firstly, details of different drilling events encountered were captured and ranked against each formation in terms of their severity. Secondly, 1D-Mechanical Earth Models (MEM) were prepared to develop an understanding of in-situ stresses with rock elastic and strength properties. Shear failure (breakout) gradient was then estimated corresponding to each lithofacie. Finally, analysis mentioned above was checked in corroboration with the drilling parameters like ECD, flow rate, stand pipe pressure, torque, drag and mud properties to establish the root cause of NPTs. Wells are 50-60deg deviated and drilled through formations consists of sand, shale and limestone lithofacie sequence. Study reveals that high well deviation and characteristic hard and soft lithofacie behavior of Godavari Clay and Matsyapuri sandstone formation were the main reasons for different drilling challenges. Shales are observed to be considerably weak with rock strength in the range of 1000-3500psi and required higher mud weight 1.20-1.35gm/cc to minimize breakouts. On the other hand, sands are permeable and relatively strong with rock strength ranging 3500-8000psi. High mud weight to control cavings due to shear failure in shales invite high differential pressure of 0.3gm/cc and associated risk of differential sticking in sands. In addition, at high inclinations the shear failure gradient is further higher and further worsening the situation.

Study concludes that differential pressure, cavings due to shear failure of rock and poor hole cleaning are key factors responsible for the stuck pipe incidents. Optimization of mud weight, bottom hole assembly (BHA) design, mud fluid loss property, and hole-cleaning practices that can overcome these challenges. Specific recommendations are made to allow partial collapse of shales by using a lower mud weight which minimizes the risk of differential sticking in sands.

In addition, proper hole cleaning practices are discussed to prevent the accumulation of cavings and cuttings beds on the bottom of the borehole. Drilling and geomechanics insight into the offset wells with their sidetracks helped operator in drilling future exploratory deviated wells in the area.