Case Studies of Tight-Gas Carbonate Reservoir Stimulation: Acid Fracturing vs. Propped Fracturing
Authors: Li Xianwen , Zhao Zhenfeng , Ma Xu , Wang Xiaoduo , Zhou Huanshun 
Published: December 2007

Abstract
This paper presents a detailed case history of hydraulic fracture treatments in tight gas wells in a remote onshore location of China. The target formation consists of heterogeneous dolomite deposits and occurs at various depth ranging from 3,100 to 3,700 meters, with an average net pay of a few meters, porosity of about 4%, and effective permeability of &lt;0.1 mD. All the wells in the region were fracture stimulated in an attempt to achieve commercial production and to improve productivity. Because high Young's modulus and high net pressure were observed, placing proppant inside fractures was always a problem and acid fracturing was the choice of stimulation in the past. With acid fracturing treatments, mixed stimulation results were achieved with various acid types and design strategies. Over the last few years, propped fracturing treatments in the region were "re-introduced" with improved technologies, and 46 propped treamtnets were successfully conducted. The propped treatment size increased from 9 tons of proppant with an average proppant concentration of 120 kg/m3 in the early trials to a record size consiting of 56 ton of proppant with an average proppant concentration of 460 kg/m3. This paper provides a brief summary of reservoir background and past development history, but focuses on acid and propped fracture evaluation. A comprehensive data set of logs and post-fracture production results were compiled and employed to understand fracture performance in this complex reservoir environment.

Introduction
The dolomite reservoir is located in the Jingbian gas field, Northwestern China. The temperature of the gas-bearing formation is about 105°C. The reservoir isinitially slightly under-pressured with an average value about 32.4MPa. TheLower Paleozoic Mawu l isthe gas interval, in which Mawu13is the main paywith its air permeability of 0.01–5.0md. Acid fracturing is the inevitable technique for completion in this area.
Initially one of the major stimulation techniques was acid fracturing. Conventinal acid (20%HCL), gelled acid and LCA (Leakoff Control Acid) were adopted as the main stimulation fluids. Those fluids were used in certain kinds of reservoirs with favorable property, such as type I andtype II (see Table 1) and provided good stimulation results. Therefore, acid fracturing became the agent technologyfor developmentin the Jingbian gas field.
With the unending development of the gas field, reservoir conditions became worser and the numbers of wells in the category of typeIII reservoir increased with years. A fair proportion of wells had low production rates after acidfracturing, which could not reach commercial production. There have been reported that the effective penetration of acid fracturing was too short to meet the demand of stimulation in tight gas reservoirs [1, 2, 3]. In order to get longer effective penetrationand increaseproduction rates, hydraulically propped fracturing has been considered [4]; design study and on-site trial have been put in practice.
Since 2000, the propped fracturing designs have been improved continuously. In the early phase of project, fracturing jobs had been conducted with small-sized treatments in12 wells due to the obstacles of screen-out. The problem of screen-out had been resolved and effectively applied in 34 wells in 2005. In summary, the development of stimulation technology has experienced threephases.
During the period of 2000 to 2002, acid pre-treatment were used to decrease the formation breakdown pressure. Considerations of carbonate fracturing on proppant and fluid selection were the same as chosen to sandstone fracturing. Screen-outs happened in early stage of the treatments in 4 wells with proppant volume less than 12 tons and concentration from 80 to 122Kg/m3.
From 2003 to 2004, the fracturing fluids had been adjusted to improve fluid rheology. And, the combined borate and zirconium cross-linkerhad been usedrather than the single borate cross-linker. Six of eight frac jobs were performed successfully with the total proppant of 32 tons and average concentrationof 444Kg/m3. The success rate of carbonate fracturing had improved; however, the conventional fluids used had caused the problems of gel break-down and fluid flowback.