Journal of Engineering Geology, Vol. 9, No. 2, Summer 2015

New Analytical Approach for Reservoir Stress
Approximation Based on Acid Fracturing Data
Haghi A.H.; Dept. of Petroleum Eng., NISOC, Ahwaz
Kharrat R.; Petroleum University of Technology
Asef M.R.; Faculty of Earth Sciences, Kharazmi University,Tehran, Iran
Received: 3 Sep 2013 Revised: 11 Nov 2014
Abstract
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Downloaded from jeg.khu.ac.ir at 11:34 IRST on Saturday October 28th 2017 [ DOI: 10.18869/acadpub.jeg.9.2.2789 ]

In this research attempts were made to estimate the in-situ stresses acting on a hydrocarbon reservoir based on routine activities of acid injection in oil reservoir. It was found that the relation between the reopening pressure of fracture and principal in-situ stresses can be estimated using rock mechanics equations for the circular underground cavities. An appropriate relation between the maximum and minimum horizontal principal in-situ stresses and reservoir parameters such as permeability, reservoir pressure, Young’s modulus, acid viscosity, injection flow rate and etc., was developed by using the well-known Darcy equations for fluid flow in porous media. Accordingly, knowing the flow rate of acid injection during well operations and some other reservoir parameters, in-situ stresses may be estimated. The method was then successfully applied to a large carbonate reservoir as a case study in south-west of Iran. Maximum and minimum effective horizontal stresses were calculated by
employing the presented method.
Keywords: Rock mechanic; Acid fracturing; Re-opening pressure; In-situ stresses.
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1. Introduction
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In-situ stresses acting on a hydrocarbon reservoir are essential characteristics to be known before any geomechanical evaluation is completed. Knowledge of stress at smaller basin and field scales has critical importance for petroleum applications such as wellbore stability, and reservoir compaction [1-3]. Although some methods have been introduced for direct measurement of these stresses, practical limitations such as finance, time, and accessibility at the time data is required, encourage industry to seek for alternative options. In general, two different approaches may be followed for this purpose. First, laboratory experiments on core specimens such as differential strain core analysis DSCA [4], differential wave velocity analysis, DWVA [5], and an elastic strain recovery (ASR). These methods can be used if the direction core specimens are known [6, 7]. Second, field approaches based on down hole measurement as follows seem to be more reliable, but also more expensive:
Hydraulic fracturing [8],
Study borehole break-outs and subsequent elongation plus the natural fracture pattern around the borehole by well logs like caliper log [9,10],
Study drilling induced tensile fractures by formation micro imager, FMI, and/or borehole tele-viewer logging, BHTV [11-
13],
Over coring and focal mechanism [14-16]
Further information on the subject could be viewed also in the related literature such as Fjaer et al. [17] and Zoback [18].
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Furthermore, accumulation of data in oil-rich states encouraged some authors to determine the magnitude of the in situ stresses. Reynolds et al. [19] determined the magnitude of the in situ stresses in the Cooper-Eromanga Basins using an extensive petroleum exploration database from over 40 years of drilling. They calculated the vertical stress based on density and velocity check-shot data and used leak-off test data for estimation of the lower bond of minimum horizontal stress magnitude, and closure pressures from a large number of minifrac tests for estimation of the minimum horizontal stress. They reported that the magnitude of the maximum horizontal stress was constrained by the frictional limits to stress beyond which faulting occurs and by the presence of drilling-induced tensile fractures in some wells. Raaen et al [20] promoted the current extended LOT for estimation of minimum horizontal stress by adding a monitored flowback phase. With respect to hydraulic fracturing and Hydraulic test on pre-existing fracture a full explanation was presented as ISRM suggestions by Haimson and Cornet [21]. Still fracturing technologies have inefficiency in SH estimation in oil fields [22].
In this research, acid fracturing method is presented as a stress indicator. An appropriate relation between the maximum and minimum horizontal principal in-situ stresses and reservoir parameters such as permeability, reservoir pressure, Young’s modulus, acid viscosity,
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injection flow rate and etc, was developed by using the well known Darcy equations for fluid flow in porous media. Through presented method in this research, breakdown pressure is substituted by reopening pressure in order to eliminate inaccuracies in detecting breakdown pressure.
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Knowledge of the present-day stress orientation is particularly important in Iran, which has an extensive and mature petroleum exploration and production industry. Yet, the 2008 World Stress Map database contains very little present-day stress information for Iran and no stress data from petroleum wells [23]. Yaghoubi and Zeinali [24] investigated a detailed profile of the stress orientation in two wells in the Cheshmeh Khush oilfield in SW of Iran. Later, Rajabi et al. [25] examined resistivity image logs and determined the presentday stress orientation of the Abadan Plain in SW Iran. Recently, Haghi et al. [26] conducted an analysis of the present day stress of the central Persian Gulf using full-bore FMI log, leak of test and density logs. By creating the first full stress tensor, they concluded a strikeslip stress regime in the studied area in the South of Iran. These researches indicated that the stresses in South and South-West of Iran are linked to the resistance forces generated by the Arabia – Eurasia collision at the Zagros orogeny.
In this paper, present day insitu stress is investigated based on acid fracturing data for a carbonate reservoir in SW of Iran. Using this method, maximum and minimum effective horizontal stresses are calculated. The calculated minimum horizontal stress is compared with leak-off pressure, and this comparison validates the accuracy of the introduced method.

2. Acidizing Process and Insitu Stress
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Acids are used as fracture fluids, for scale removal as well as matrix treatments. Acids are also used to clean up gravel packs once they are positioned, or as cleansing agents to preflush the formation prior to administering a near-wellbore chemical treatment. Acid systems in current use can be classified as mineral acids, dilute organic acids, powdered organic acids, hybrid (or mixed) acids and retarded acids. All acids with the exception of hydrochloric-hydrofluoric and formic-hydrofluoric acid mixtures are used to treat carbonate formations. It is, with few exceptions, generally necessary to include hydrofluoric acid (HF) in treatment of sandstones [27].
In addition to HCL there are other organic acids used to treat carbonate formation, HCL is one of the most ordinary in acidizing. As a result, in this paper only the typical reactions of HCL are mentioned as follows:
2HCLCaCO3(calsite)CaCl2 H2OCO2
4HCLCaMg(CO3)2(dolomite)MgCl2 CaCl2 2H2O2CO2
Acidizing treatments in sandstone formations normally employ a mixture of HCL and HF. This acid mixture is used because HF is
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reactive with clay minerals and feldspars that may be restricting the permeability near the wellbore.
2.1. Acid Fracturing:
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In oil industry acidizing is a well-known method for well stimulation. Accordingly, creating a fracture by an acid fluid in an area around the well will effectively remove the skin effect. As a result, hydrocarbon will bypass the damages zone through fractures rather than crossing through porous media. Therefore, the permeability of the area around the well will increase, pressure drop related to skin effect turns to negative value and consequently the productivity index of reservoir will increase. The following equation is typically used for analysis of the this process[28]:
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