Petro-Phyiscal Evaluation Methods of Complex Clastic Deep Tight Gas Reservoirs to Improve Hydraulic Stimulation Efficiency
Authors: Al Hinai Adnan Saif , Abdelazim Mohamed Fathy , Al Aamri Mohammed H. , Al Wahaibi Ahmed Mahfoudh 
Published: March 2022

In most cases calstic deep tight gas reservoirs contain considerable hydrocarbon reserves but the ultra-low permeability and the poor inter-connection between the pores may dramatically reduce the recovery for economical gas production. In such cases, hydraulic stimulation techniques can be beneficial to improve the connectivity between the pore spaces and provide a larger conductive channel to allow communication between the reservoir and the well bore. Prior to the fracturing design it is imperative to understand the borehole and the reservoir environment. Hence, the application of petro-physical evaluation.

The main objectives of this paper is to discuss the petro-physical evaluation; conventional and advanced methods to understand fracture initiation and propagation behaviors that are essential to plan, design and execute an effective hydraulic fracture treatment program. A large range of wireline logging tools are available today, and several provide sophisticated interpretations of the formation; including details of formation lithology, fluid type, porosity, fluid content and stress regime, etc.

In this paper, integrated interpretation has been conducted from both open and cased hole wireline logging. With the aim of assessing the rock mechanics, formation pressures, cement bond, well bore integrity and other reservoir properties to establish a prominent fracturing zone in Barik and Miqrat tight gas reservoirs.

Post fracture analysis such as radioactive tracers has been demonstrated, production logging and noise log are studied and linked with the amount of proppant placed in the selected fracturing intervals to assess the stimulation efficiency.

Cement bond log evaluations showed good zonal isolation across the 4.5" tubing in the studied wells, notably in Barik and Miqrat reservoirs. However, poor to intermediate cement was observed across the overlap section between 9 5/8" and 4.5". In which was not a stimulation obstacle.

Saturation and electrical parameters were derived from Archie's equation and Special Core Analysis (SCAL), respectively. The interpretation revealed that Barik and Middle Miqrat formations are relatively conclusive in some fields and not in others. Mainly due to the high saturation of the trapped gas due to the tightness of the reservoir. Moreover, based on the saturation log analysis, porosity controlled hydrocarbon saturation profile and created a challenge in determining the top of water bearing interval. Non-resistivity based saturation estimates, such as pulsed neutron and dielectric logs did not offer benefit in fluid typing.

Well test showed different results as compared with the anticipated water and gas rates. Gas inflow was observed in all tested wells. In addition, some wells not-necessarily located in the extreme flanks of the field, showed high water influx. Irreducible water saturation derived from NMR and/or capillary pressure data helped to identify moveable water in Barik but not in Lower Miqrat formation due to presences of bitumen and vugs.

Selective completion strategy for hydraulic stimulation proved to be successful by screening the reservoir intervals thru the use of a combination of petrophysical and cased hole production analysis. Allowing hydraulic fracturing execution to achieve up 90% of the desired proppant placement.

Knowledge of in-situ stresses (magnitudes & directions) is critical to understand hydraulic fracture initiation & propagation behaviors. The initiation and propagation behaviors are essential to plan, design and execute an effective hydraulic fracture treatment program. Open hole and cased hole logging are key practices for evaluating fracture behavior. It provides the grounds to optimize for future wells for stimulation.