Complex Hydraulic Acid Fracturing Field Application in Appraisal and Development of Tight, Sour, Over-Pressured, and Tectonically Active, HPHT Carbonates in North Kuwait

Abstract

Producing hydrocarbons at appraisal and development targets from deep, overpressured, high pressure/high temperature (HPHT) Jurassic carbonates in Northern Kuwait has been a challenge as a result of the complex reservoir heterogeneity. Because of the tight carbonate formation properties, matrix acidizing does not always deliver hydrocarbons at economical rates; in this scenario, hydraulic fracturing is required. Hydraulic fracturing, however, presents placement and activation challenges as a result of the wellbore construction limitations and a complex tectonic setting/high stress environment. The zone of interest in this dolomitic reservoir was identified as an acid fracture candidate because of the immobility of fluids identified during multiple pressure sampling tool attempts, despite a reasonable valuation of the log-computed porosity and permeability in the range of approximately 10% and 1.1 md, respectively. In addition, solid hydrocarbons (bitumen) were reported in the cuttings samples, which indicates the possibility of in-situ conductivity damage. The well trajectory was designed as a high angle deviated well to maximize the reservoir exposure, with a maximum inclination of 49 to 50° through the zone of interest. The reservoir was drilled at a slight overbalance with 13.0 ppg of oil-based mud, using mud weight management techniques to minimize the formation damage. The highly deviated wellbore and the highly anisotropic stress regime complicated the effective design and placement of the acid fracturing treatment. As an added challenge, the operation had to be completed within a very short period because of the high cost of the deep drilling rig on site to facilitate the operation. The hydro-fracture field case was built on the analyses of the open hole logs, 1D geomechanical earth model, and a customized data-fracture suite to meet the data acquisition needs, followed by the design and calibration of an acid hydro-fracture treatment using a pseudo3D grid-based fracture modeling and calibration software. A solids-free dynamic diversion schedule was built and field-laboratory level fluid design tests were conducted to reach the most optimal design possible. A robust and operationally pragmatic fracture program was developed and implemented successfully in a very short notice period. The mobility of the formation fluids was established, leading to a critical understanding of this sour unconventional carbonate flow unit. Data fracture analyses and a customized acid fracturing technique described in this paper are the first of its kind in the deepest parts of the Northern Kuwait sour gas basin. A collection of completions data has proven critical in terms of reservoir deliverability aspects and in the calibration of the mechanical formation properties, leading to a better understanding of the hydro-fracture geometry and how to effectively connect to the higher mobility segments of the reservoir. This paper also outlines the future optimization plans based on the lessons learned from the fracture tests conducted in the well.

Analysis of Flow Behavior for Acid Fracturing Wells in Fractured-Vuggy Carbonate Reservoirs

“…Therefore, wormholes, together with artificial fractures, form the main channels of the fluid flowing into the wellbore. At present, acid fracturing has been widely implemented to enhance productivity of wells with damaged zones or produce from tight carbonate reservoirs [23][24][25]. Unfortunately, complex seepage systems created by this stimulation pose challenges to study the transport processes in underground natural-resources recovery.…”