Hydraulic Fracturing Case Histories in the Carbonate and Sandstone Reservoirs of Khuff and Pre-Khuff Formations, Ghawar Field, Saudi Arabia

Abstract

As part of gas production enhancement and quick recovery of reserves initiative, hydraulic fracturing campaign was initiated in 1998 to treat the sandstone and carbonate formations in the Ghawar gas fields in Saudi Arabia. To date, numerous wells have been matrix acidized, acid fractured, and proppant fractured. Many different procedures and fracturing techniques have been adopted as part of learning process to optimize candidate selection, perforation strategy, fracture design, and implementation.1,2 The incremental production from the Khuff carbonate gas ascertains the success of the acid fracturing treatments.3 For the Pre-Khuff sandstone reservoirs, because of the unconsolidated nature of some high potential sandstone layers, indirect fracturing technique has been adopted where the consolidated, low porosity section is perforated and from where the induced fracture is initiated.4 This procedure curtails the risk of sand production. Very recently, fracturing wells with frac&pack technique and completing them with mechanical screen are being implemented. Successes on these Pre-Khuff treatments are usually judged by sand free production rate and conditions and by comparing post-fracture production increase with simulated, pre-fracture production based on reservoir properties calibrated by post-fracture production match. In most cases this gain is significant to justify the expenses of the fracture treatment. However, the priority in Saudi Aramco is to recover the required volume of gas to feed its giant gas plants.This paper presents field cases from the carbonate and sandstone reservoirs treated with acid and proppant, respectively and provides a general picture of how all the fracture treatments are currently being conducted. Introduction Gas wells in the Ghawar field are currently fractured to enhance gas recovery and feed the present and upcoming giant gas plants. To date over 50 wells have been acid fractured in the Khuff carbonates and about 15 wells have been propped fractured in the Pre-Khuff sandstones.5 The Jauf and 'Unayzah reservoirs form the productive Pre-Khuff formation where high permeability, unconsolidated sand layers are often encountered. One major objective of fracturing sandstone is also to reduce sand production to the minimal acceptable level. This paper presents case histories from the fracture treatments. Examples are selected such that the variation of reservoir properties and different fracture techniques are represented. Due to the vast areal spread of the reservoirs and the thickness of the formations, the lateral and vertical variation of important reservoir flow and mechanical properties such as porosity, permeability, in-situ stress conditions, Young's modulus, fluid characterization, etc., change. It is therefore a challenge to optimize the treatments and come up with unique way for completion and production enhancement. The examples depicted cover the property variations and provide a comprehensive understanding on the treatment approach. Examples provided presents geological description, geomechanical property evaluation, perforation selection strategy, treatment design and execution consideration, and some post fracture evaluation. Both successes and marginal successes in fracturing treatments are presented and the reasons for either are discussed. Three-dimensional hydraulic fracturing model to simulate acid reaction and proppant transport and reservoir simulation models for production and transient data evaluation have been used for the analysis. Finally, conclusions are drawn to provide guidelines to best practices for enhanced gas recovery.

Mineralogy Impact on Acid Fracture Design in Naturally Fractured Carbonates

“… 4 Also, both minerals show deterioration in the mechanical properties after acid injection to different degrees. 25 , 26 In addition, dolomite could act as frack barrier as usually, it is more stressed than limestone; however, we assumed similar closure stresses. Also, the stress acting on NFs was assumed to be 20% larger than the closure stress acting on the HF.…”

A Novel Pore-Scale Thermal-Fracture-Acidizing Model With Heterogeneous Rock Properties

“…The temperature range considered in this study is from 300 to 339 K, following Settari (1993). Over this range, the rock (calcite) thermal conductivity varies by approximately 10% (Robertson 1988). Also, the fluid (water) thermal conductivity varies by approximately 10% (Lemmon et al 2010).…”

“…Therefore, both conductivities are then assumed to be constant in this study. The specific heat capacity of water, c p , is approximately constant (Robertson 1988). The reaction rate, as a function of temperature, can be calculated by use of the Arrhenius equation:…”

Acid Fracturing Tight Gas Carbonates Reservoirs Using CO₂ to Assist Stimulation Fluids: An Alternative to Less Water Consumption while Maintaining Productivity

“…Also, because of this nature, the acid spending rate appears to be slowed, even though the formation is 280 to 300°F (Zillur et al 2002). The Khuff formation is a deep gas carbonate reservoir that consists of dolomite and limestone sections underlying the giant Ghawar field in the eastern region of Saudi Arabia.…”