Major gas fields in West Siberia have been depleted by 50-80%, and field operators are actively searching for ways to reverse declining production and access new gas deposits previously ignored given their complexity. One such hard-to-recover deposit is the Turonian play. With total gas reserves reaching approximately 3 trillion cubic meters, Turonian gas is encountered in nearly the entire Northern part of West Siberia.
Developing the Turonian deposits presents the following challenges:
Poor reservoir properties and low well productivity;
Significant reservoir heterogeneity and facial variability along the section and area-wise, which complicates well placement operations for an optimal gas extraction;
Low formation temperature and abnormally high pressure, which may require well operations in a near-hydrated regime;
High clay content, which leads to formation damage when water-based fluids are being used in reservoir drill-in operations.
Drilling of Turonian horizontal wells is complicated by a narrow safe mud weight window, geology uncertainties, and high sensitivity of borehole stability to well trajectory.
Analysis of Turonian well construction experience has identified technological solutions in two main areas:
Provide high-quality hole in all sections of horizontal well, including lateral sections, with length more than 800 m (to run safely casing to planned depths with followed multi stage hydraulic fracturing in horizontal section).
Minimize formation damage, thereby preserving potential well productivity in a formation with a high clay content in wide range of temperature and pressure conditions.
Core samples from the Turonian formations were tested to determine mechanical, elastic, and other rock properties. From these results different mud types were evaluated for formation damage and impact on the mechanical and elastic rock properties.
The integration and collaboration of drilling and petro-technical experts was the key to developing a fit-for-purpose drilling system that combined drilling technologies and engineering solutions. The main outcome from the planning stage was joint development of the engineered drilling system (EDS), where each component (rig, surface and downhole equipment, drilling fluids, wellbore stability, trajectory, and engineering practices) was considered equally important and influenced the entire system and results of the well. Implementation of the EDS approach resulted in successful well drilling and completion operations.
A time-dependent wellbore stability model was developed and successfully tested in field operations. Implementation of the EDS approach helped to determine the importance of time on the stable operation window for mud weights. Additionally, the EDS approach helped achieve production rates significantly higher in offset wells and justified the project's economics.
The key lessons learned can be applied in other fields in the region to develop shallow low permeability formations. The major of them are:
Geomechanics is a vital part for productive drilling;
Optimal selection of dril-in and fracturing fluid played a critical part in achieving the project objectives;
Integrated approach to drilling system design and close cooperation between all team participants are the key success factors in delivering technically complex projects;
The combination of fit for purpose technologies into an effective drilling system can provide cost effective solutions for field development with region specific challenges and traditional technology limiters.