Deep Horizontal Drilling Optimization in a Highly Fractured and Dense Formation.
Authors: Vasquez Bautista Ramiro Oswaldo , Al Lawati Ali Baqir , Heidari Varnamkhasti Mohammad Reza , Ali Al Riyami Aktham , Al Kendi Mahmood , Wahshi Khalifa , Manthari Tariq , Burashdi Said , Shaibani Hamed , Maqbali Salah , Rubaiey Hilal , Al Balushi Humaid 
Published: October 2019

In 2017, Petroleum Development of Oman (PDO) initiated a project to continue developing the Y gas field located in northern Sultanate of Oman. This field consists of deep horizontal producer wells in the Limestone-K1 limestone formation. The project began with a project completion time of 181 days. The objective was to optimize the well design, bottomhole assemblies (BHAs), and casing points to optimize drilling operations and reduce costs.

Due to the 17.5-in. and 12.25-in. sections being drilled vertical, the main optimization focus of the project began in the 8.375-in. buildup section and the 6-in. lateral section. A motorized high-build rate rotary steerable system (RSS) was used in the 8.375-in. section, which allowed for drilling the section in one run, including building from 0 to 90° inclination, crossing interbedded formations with unconfined compressive strength (UCS) of 25 to 30k psi and 2.8- to 2.9-g/cm3 bulk density. This engineering design achieved the required 5.5°/30-m dogleg severity (DLS) and reduced the drilling time for the 8.375-in. section from 17 days to only 3.5 days, The 6-in. lateral section was the most challenging due to trajectory control issues in the Limestone-K1 formation (2.4- to 2.8-g/cm3 bulk density), which is highly fractured with multiple faults crossing the horizontal section. This condition generates high microDLS, which increases tortuosity and torque on the drillstring, as well as stress, shock, and vibration on the tools, all of which limit the drilling performance. The 6-in. lateral section was also drilled with a motorized high-build rate RSS system. Based on finite element analysis (FEA) modeling, using the RSS reduced the number of days required to drill the section from 15 to only 6 days on a 900-m lateral.

A slim tool was designed with the objective being to reduce the well design by drilling the entire Build Up Section (BUS) and the lateral in a single 6-in. operation. This goal required an engineering approach to modify the casing point, modify control losses and trouble zones, and a special trajectory design on the BUS due to low-DLS zone output.

In the horizontal section where the formation is highly fractured, the directional control and geosteering was a challenge. The new robust and seal-free push-the-bit motorized RSS technology was proposed as a solution to the challenge. This design minimized the microDLS by 63% and stick/slip by 70% compared with a high-buildup rate RSS (using the same bit). In addition, the tool steering ratio (tool power) was reduced by 35%; thus, improving directional control.

In deep environments (3000-m true vertical depth (TVD)), obtaining a good understanding of drilling data from downhole tools, validating data through robust simulation software to apply correct drilling parameters in future wells, and interpreting data by drilling experts are instrumental in achieving the drilling optimization results, which reduced the Best Composite Time (BCT) from 181 to 63 days. Trial and error is a costly option and not acceptable when well delivery times are tight.