Fracture energy release and size effect in borehole breakout

A study on sand production forecast is essential in order to determine whether sand control measures should be installed. The emphasis of this written work has been on defining geomechanical parameters of rock and hole stability in sand production, as well as analyzing optimal hole perforation and spacing.

Introduction

Tests on the stability of a specific hole in the presence of anisotropic stress have been carried out in limestone (Haimson and Herrick, 1989), granites, and sandstone (Haimson and Song, 1998). The main concern of these experiments was on lateral stress on the failure of a borehole and also on breakthrough angle and depth. In most cases it has been proved that the rock do fail at first when drilling is in progress. When the shear or tensile stress of the rock goes beyond the allowed limits, failure occurs. After failure has occurred a raise on plastic zone distance and that of drawdown pressures are directly related up to a point where a critical value is attained.

Geo-mechanical parameters are established from the following types of tests; triaxial tests which are mainly conducted to establish the stiffness of the rock and also its characteristics that determine its strength. The other tests are Thick walled Cylinder, which is applied in calibration of numerical model (Morita, 1994). In Thick Walled Cylinder Strength tests, the sample is subjected to an external isotropic pressure which is increased gradually until failure occurs whereas atmospheric pressure is kept constant in the inner hole. The applied pressure corresponding to the pressure at which the failure occurs is called Thick walled Cylinder (TWC) strength, which changes subject to the size of the sample under test and also the ratio of outer diameter inner diameter (Morita, 1994). On the other hand, in triaxial tests, shear failure is induced by increasingly varying axial stress. These tests are conducted at varying confining pressures. Apart from rock stability, these other critical factors account for the stability of the borehole. They include overburden stress, formation pressure filed values and borehole orientation as far as in–situ stresses is concerned (Ismail, & Babu, 2004).

The main concern to analyzing how stable a borehole is is to establish if the surrounding rock, with assistance of mud pressure, is capable of withstanding the in situ stress that was initially sustained by the rock which resided in the hole. Approximating the least pressure of the mud necessary to prevent the collapsing of determined by three unique methods which are, elastic behavior of the given sample core, an empirical way with use TWC collapse test and elastoplastic behavior of sample cores. These methods are under the assumption that strength of the formation rock represents strength properties of sample cores. These methods are briefly explained below (van den Hoek et al., 2000).

Elastic model works under the assumption that hole collapse takes place after stress state at a defined point on the wall of the given borehole deviates from the criteria that prevents failure occurrence that is determined by the parameters of the triaxial strength. Table 1 below gives a summary of the least weight values of mud required to stop collapse of the borehole as predicted by elastic/brittle model.

ELASTIC/BRITTLE MODEL PREDICTIONS

FORMATION

APPROXIMATE

DEPTH (ft)

DEVIATION ANGLE (degrees)

MINIMUM MUD

WEIGHT (psi/ft)

OVERBALANCE

(psi/ft)

Middle Ness

9,000

55 to 65

0.59 to 0.61

0.05 to 0.07

Etive

9,100

65 to 70

0.77 to 0.77

0.23 to 0.23

Rannoch

9,200

70 to 89

0.39 to 0.41

-

Note: Azimuth angle = 189o , formation pore pressure=0.54psi/ft, maximum mud weight to prevent extension factures = 0.72.

TWC Empirical way is a method that assesses how stable the borehole is taking into consideration TWC strength to investigate the stability of both the horizontal and vertical boreholes. In this method, the criteria for failure are determined as follows; if the TWC strength is exceeded by the effective vertical stress, the horizontal holes will collapse and if TWC strength is exceeded by effective horizontal stress, the vertical holes will definitely fail (Ismail, & Babu, 2004). Taking into account that TWC test was done with no support pressure to the inner hole, then criteria above is only valid for drilling when the conditions for mud-weight are balanced when its ok to make a comparison between effective field stresses to TWC Strength. The third way involves an elastoplastic finite element that entails material-strain softening and hardening.

According to van den Hoek et al., (2000) sand prevention uses techniques to reduce production of sand and it achieve this by optimizing perforating practices when risk of sand production is low. However when sand production risks are high, sand control methods are adapted instead. Sand production in a borehole depends on factors for instance stresses of the perforation, strength formation, fluid type and also flow rate. When formation are very weak to an extent that they cannot support a perforation tunnel, then they not a preferred candidate for sand prevention method. Sand production mostly happens as a result of either collapse of the tunnel or if there is sand in the tunnel that is loose (Denney, 2003). Factors that lead to the loose sand being produced in the tunnel include varying rates of flow related to pressure drawdown.

It has been noted from various studies that for a borehole to produce a given amount of sand the perforation tunnels fails first. Also, flow rate has to be enough to facilitate the fluid to carry the sand that has failed. Hence the formation stress which is controlled by depletion and drawdown accounts for the failure occurrence.

The stability of the perforation tunnel plays a critical role in determining if production of sand will be an issue after the borehole has been completed. Further, when failed zones are interlinked near neighbouring perforations, it can result to collapse of formation structure and also induced production of sand. It should be noted that how stable the formation is, perforation under consideration and the ability to precisely predict them plays a key role in determining if sand prevention or sand control strategies should be done (Denney, 2003).

Tunnel Stability Perforation

As illustrated in the figure 1 above, for a certain borehole shot density and radius, optimal phasing maximizes the distances (L1, L2 and L3) between perforations.

Various methods do exist which tend to predict stability of the tunnel perforation. Commonly applied predictions do have basis on historical production experiences of wells residing in same reservoirs. In some cases the strength of reservoir data can be applied in identifying same field of which its history of production can assist in predicting sand production potential (Bažant, Lin, & Lippmann, 1993). Other ways for tunnel stability production include theoretical models that are extensions of models initially designed for stability of the borehole. The theoretical three step processes for modelling takes into account establishing mechanical properties of the rock incorporating core samples and log data, determining the environments of in-situ stress and approximating conditions for failure of the perforation. Two theoretical models direct great attention on various failure mechanism:

Tensile stress basically from flow of fluid into the cavity of perforation

Shear stress in regard to assumption of the behavior of the material

Bybee, (2004) notes that models for tensile failure are less commonly used and the Mohr-Coulomb is the most commonly applied shear failure model for predicting mechanisms of failure and conditions for shear stress. Experimental ways are applied in testing two things; outcrop rock or samples of the core that possess mechanical properties that are same to the reservoir aimed for completion. Laboratory – testing methods are uniquely classified into two; drilled-hole tests and single shot flow and perforation tests.

During the early development stages of petroleum reservoir, it is of great importance to incorporate sand production predictions so as to assess the need for sand production. Moreover, this prediction assists in choosing the most economical sand control method (Denney, 2003). Below are some of the most common types of sand production:

Transient Sand production: This is defined as sand concentration reducing with time taking into account the conditions for well production are constant. This situation is seen during clean up after acidizing or perforation. The concentration of sand, the cumulative sand volume and the decline period do vary.

Continuous sand production: In many areas, continuous levels of production of sand are evident. The accepted concentration of sand is subject to constraints of operations with regard to capacity of separator, sand disposal, erosion just to mention a few.

Catastrophic sand production: This is the scenario in which high rate sand influx result in the well to instantly choke and/or die. Two failures in this sand production type can occur. The first one refers to sand slugs forming bridges of sand of moderate volume in tubing or choke. The other one refers to a lot of sand influx filling and blocking the wellbore.

Preventing sand production

The figure 2 above illustrates that when shot density is maintained constant, wells perforated with use of optimal phasing (the phase angle optimized from 60o to 90o) had low sand production and various problems.

In order to prevent sand production, designs of perforation must maximize the spacing of the perforated hole and at the same time minimize hole size in formation, drawdown pressure across the intervals perforated and rate of flow per perforation. Perforation practices optimization entails optimizing parameters related to operations of perforating. These parameters include:

Charge type:

Perforations of small diameter produced by deep penetrating charges are highly suitable for sand prevention. Small deep perforation tunnels in conjunction with optimum perforation spacing, they enhance to a great extent the stability of the formation that is adjacent.

Shot density:

When there is a failure and interlinking of zones around the perforation tunnels, there can result to a lot of sand production due to collapsing formation. The spacing between perforations determines how stable the adjacent formation is. Lower shot densities and smaller holes increases spacing of the perforation and the stability of the formation.

Phase angle:

Phasing too dictates spacing of the perforation. Phase angle optimization for a defined radius of a wellbore and shot density maximizes the spacing between perforations and as a result reducing or avoids interaction between surrounding rock and the adjacent perforations. This is critical because it lowers failure of the formation and risk of interlinking the zones without affecting rates of individual perforation flow rates.

Perforation orientation: Oriented perforating techniques can be applied to increase perforation stability in environments involving anisotropic stress. Knowledge distribution with in-situ stress and large formation stress difference, oriented perforating can raise stability of the tunnel by aiming the direction that is most stable.

References:

Bažant, Z., Lin, F., & Lippmann, H. (1993). Fracture energy release and size effect in borehole breakout. International Journal For Numerical And Analytical Methods In Geomechanics, 17(1), 1-14. http://dx.doi.org/10.1002/nag.1610170102

Bybee, K. (2004). The Role of the Annular Gap in Expandable-Sand-Screen Completions. Journal Of Petroleum Technology, 56(05), 44-46. http://dx.doi.org/10.2118/0504-0044-jpt

Denney, D. (2003). Enhanced Gravel-Pack Completions Revitalize a Mature Sand-Producing Field. Journal Of Petroleum Technology, 55(01), 42-43. http://dx.doi.org/10.2118/0103-0042-jpt

Denney, D. (2003). Sand-Production Prediction of a Gas Field. Journal Of Petroleum Technology, 55(03), 65-66. http://dx.doi.org/10.2118/0303-0065-jpt

Geomechanical stability analysis for selecting wellbore trajectory and predicting sand production. (2010). “Proceedings” Of “Oilgasscientificresearchprojects” Institute, SOCAR, (4). http://dx.doi.org/10.5510/ogp20100400040

In situ stress evaluation from borehole breakouts. Experimental studies. (1987). International Journal Of Rock Mechanics And Mining Sciences & Geomechanics Abstracts, 24(1), A15. http://dx.doi.org/10.1016/0148-9062(87)91348-9

Ismail, I., & Babu, J. (2004). Analysis of Formation Damage Caused by Oil–Based Mud on Berea Sandstone Using SEM. Jurnal Teknologi, 40(1). http://dx.doi.org/10.11113/jt.v40.423

Morita, N. (1994). Field and Laboratory Verification of Sand-Production Prediction Models. SPE Drilling & Completion, 9(04), 227-235. http://dx.doi.org/10.2118/27341-pa

van den Hoek, P., Kooijman, A., de Bree, P., Kenter, C., Zheng, Z., & Khodaverdian, M. (2000). Horizontal-Wellbore Stability and Sand Production in Weakly Consolidated Sandstones. SPE Drilling & Completion, 15(04), 274-283. http://dx.doi.org/10.2118/65755-pa