Much of the drilling in unconventional resource plays occurs in unstable shales, which are usually fractured and can be easily destabilized. Successful drilling through them can be difficult at best, and many high-angled holes in these plays are often lost due to mechanical instability. This paper looks at the drilling problems of shale gas drilling from the theoretical perspective of Wellbore Pressure Management, (WPM) and keys in on the effects of Equivalent Circulating Density (ECD) while drilling and on the effects of Equivalent Static Density (ESD) when there is no circulation.
In this page the following questions pertaining to drilling a typical fracture shale are addressed from the WPM perspective:
- What mud density do I need to drill a fractured shale?
- Why can a typical shale gas play well be drilled with no drilling problems, yet becomes very unstable on the last trip out of the hole before E-logging or running casing?
- Why are drilling problems especially acute in laminated shales or similar weak zones?
By using a Wellbore Pressure Management approach to understanding instability in fractured shales, the reader can readily see how to best deal with the problem in the field and hopefully improve stability in future wells.
Drilling problems in unconventional resources wells typically involve fractured or laminated shale zones. These formations are by their nature weak, and susceptible to pressure fluctuations that occur during the normal course of drilling. Turning on the mud pumps and later turning them off when a connection is made happens regularly while drilling. Other fluctuations can occur when wellbore instability becomes a problem, and poor hole cleaning, pack-offs, etc. often result. To deal with the instability issues, drillers will often rely on backreaming, which often produces pack-offs and creates additional pressure spikes. In short, the pressure fluctuations become worse instead of improving. These pressure fluctuations have previously been called ‘wellbore flexing’ or ‘pressure cycling in the literature. It is only with a better understanding of the causes and outcomes of these pressure fluctuations in unstable shales that the drilling industry can more efficiently drill these wells – hence the term ‘Wellbore Pressure Management’, or WPM for short. In Fig. 1, an outcrop of an unstable shale of the type discussed in this page is shown.
WELLBORE STABILITY MODELING
Typically, before a well is drilled, the Operator engages in a wellbore stability study in order to identify the ranges of the Safe Drilling Window. This window describes the density required to maintain stability in an interval for a given hole angle and azimuth, if the downhole formation pressures in the drilling plane are anisotropic in magnitude. Wellbore stability modeling is usually done using the downhole formation pressures
(maximum and minimum horizontal stresses), pore pressure, and rock mechanical properties for a formation at a known True Vertical Depth (TVD).
In Fig. 2, a polar chart is shown from a typical wellbore stability scenario. Hole angles in 10° increments are shown from the center toward the edge, and azimuthal direction starts from 0° north clockwise around to 360° back at true north. From this kind of modeling, the density required to maintain a borehole stable with compressive shear is identified. A second simulation can be easily done for pressures required to initiate fractures.
Multiple scenarios for the range of hole angle from 0–90° deviation from vertical can then be run to produce a typical Safe Drilling Window for an unstable shale near interval Total Depth (TD). As Fig. 3 shows, the Safe Drilling Window is bound on the top by pressure to initiate a fracture and on the low side (usually) by the pressure required to initiate hole collapse. In vertical/near-vertical wellbores, the formation pore pressure can exceed the hole collapse pressure, and thereby can serve as the lower bound on the Safe Drilling Window for these type wells.
It can be also seen from Fig. 3 that the magnitude of the Safe Drilling Window narrows with increasing hole angle, and, as in this particular North Sea case, can become quite narrow in a horizontal / near-horizontal drilling case. At 30° deviation, the window width is approximately 6lbm/gal equivalent, while at 85° deviation, it is only 0.4 lbm/gal equivalent. Hence in order to avoid hole instability while drilling this shale, it should be drilled keeping the downhole pressures within this Safe Drilling Window. The densities predicted by the Safe Drilling Window can be converted to pressure in field units (lbf/in2) by: P = 0.052 · MW · TVD （1）
In a recent work , the strength of fractured shale in particular was discussed. It was shown that fractured or laminated shale should not be modeled using peak Uniaxial Compressive Strength (UCS) value, but rather its ‘residual’ strength value, which can be significantly lower in magnitude. Because of the inherent weakness of laminated and fractured shales, laboratory strength tests are usually done on intact rock, which gives elevated strength values. Use of a ‘residual’ strength however implies some borehole compressive shear failure (breakout) is acceptable. If no breakout is allowed, then mud densities higher than the residual strength value will be required to stabilize the shale.
DOWNHOLE CIRCULATING PRESSURES
While drilling, the circulating mud system is exerting pressure on the wellbore wall. For a given drilling or fracturing fluid density (MW), the fluid
is circulated through the wellbore, consuming extra pressure as needed to push the fluid up the annulus. This extra pressure is frictional pressure, and at the bottom of the hole is added to the system MW to give Equivalent Circulating Density (ECD). If a compressible drilling fluid (oil-based or syntheticbased fluid or the like) is being used, then the fluid compressibility and thermal expansion should be taken into account, depending on the downhole circulating temperature and pressure profiles, in order to determine the actual downhole MW, or what is usually called the Equivalent Static Density (ESD). The resulting ECD is the pressure exerted by the mud column on the wellbore wall while circulating, and the ESD is the pressure exerted by the mud column when the fluid is static. There is always a differential between ECD and ESD caused by the friction at the conduit walls during circulation.
In Fig. 4, the consequences of ECD on the Safe Drilling Window shown in Fig. 2 are shown. With an ECD level of approximately 13.5 lbm/gal equivalent, circulating pressures would be high enough to initiate fractures if the formation were drilled at an angle of 70° or higher.
As a result, good WPM techniques should be used to drill through this problem shale at a high angle of deviation, or the wellpath should be changed in order to penetrate the shale at a lower angle where the Safe Drilling Window is wider.
MECHANISMS OF FAILURE IN FRACTURED OR LAMINATED SHALES – A LITERATURE REVIEW
Over the years, many researchers have investigated the problem of shale failure and collapse in fractured and/or laminated shale/siltstone sequences. A brief summary of their cited causes for failure are listed below in chronological order:
- Investigators reported increases in both drilling fluid density and drilling fluid rheological properties reduced borehole instability problems. The authors also claimed that increases in drilling fluid density opens radial fractures and closes tangential fractures.
- Increases in drilling fluid density accelerates poor hole conditions by increased fluid invasion into the cracked rock, which reduces the rock bulk strength and lubricates the bedding planes. In addition, they found the formation samples from the zone of instability to be chemically unreactive.
- Wellbore instability problems occurred in Extended Reach Drilling (ERD) wells in Alaska, and were associated with drilling mud losses and gains in tuffs and shales above the reservoir. They listed as causes of the instability: 1. drilling-induced radial fractures; 2. ECD-induced circumferential tensile
fractures; 3. pre-existing faults and fractures. They also reported increasing fluid density after stability problems occurred, and sometimes no improvement was seen after increasing the MW.
- Pressurizing any fractures in shale with the drilling mud should be avoided, as the wellbore wall becomes pressurized to a level equal to mud density. The resulting increased pressure in the microfractures does not bleed off into the rock matrix. Also, they showed use of materials to plug the throats of microfractures at the borehole wall can reduce wellbore instability
problems. Any drops in wellbore pressure (stopping circulation, swabs, etc.) can induce cavings into the wellbore.
- Chemical and mechanical disturbances in the wellbore can cause wellbore instability problems . In this work, the authors concentrated most on the chemical effects of osmotic pressure generation of invert emulsion drilling fluids on shale strength. They did cite a case (Well A) where increased drilling mud density had a negative effect on wellbore stability.
- The greatest wellbore stability problems occur when drilling at an angle close to the angle of bedding planes (0-30° angle of attack) These planes of weakness are not very important to wellbore stability in vertical wellbores as drilling is usually nearly normal to the bedding planes.
- Laminated shales are anisotropic in strength, a concept similar to the planes of weakness cited immediately above. Wellbore failure was exacerbated by mud invasion into the fracture planes. They reported increases in MW were needed to prevent bedding plane failure. They also
found excessive pressure fluctuations were detrimental to wellbore stability and shale cavings often occurred while pulling out of the hole (POOH).
- Insufficient MW caused wellbore stability problems in a hole drilled with an invert emulsion, and the situation improved on the following sidetrack where increased drilling fluid density was used. A more-gauge hole caliper showed positive effect the higher density had on hole stability. Laboratory
testing on a core of the fractured shale showed the intact rock strengths were higher than the residual strengths (eg, shale anisotropy as in Ref. 8). Moreover, drilling fluid and pressure diffusion into the fracture network can occur, which can lead to further instability. While plugging or sealing agents can be used to reduce the pressure diffusion, alone they will not promote stability.
- Naturally-fractured shales can have fractured widths as low as 10-6 m, and their networks can extend throughout a shale sample. The difficulty of handling and testing the strength and chemical reactivity of these shales has been described, with accompanying photos that make the complexity of the problem easier to understand. The authors then showed early work on the failure of these rocks using poromechanical modeling.
After a summary review of previous work in this area, the following are key factors in understanding the instability of fractured shales:
- There has to be a zone of weak rock, either a fractured shale or laminated zone. These reports of instability generally do not occur in intact shale. Moreover, this zone of weak rock can lie anywhere in the open hole, not necessarily at the bottom of the wellbore.
- The shales are not chemically reactive, and hence have low smectite content and low water content. These shales are not plastic and show relatively low interaction with water.
- The existence of bedding planes exacerbates the problem, especially if the borehole is drilled at a low angle of attack in reference to the bedding planes.
- There has to be a microfracture or fracture network in the shale zone through which drilling fluid and especially pressure can be transmitted.
- Pressure fluctuations are required to destabilize the shale. These fluctuations can come from several sources: a: Pressure cycling (mud pumps on/off); b: ECD reverting to ESD, especially on trips out of the hole; c: Backreaming and/or surge/swab; d: Excessive hydraulics for hole cleaning; e: Excessive drill pipe rotation speeds, especially in narrow annuli.
PROPOSED SCENARIO FOR INSTABILITY
In Fig. 5 and 6, the proposed scenarios for the instability in these shales are drawn. The laminations and/or bedding planes are drawn horizontally in this diagram (but do not have to be necessarily horizontal in the field). Fig. 5 represents the normal drilling scenario when the mud pumps are operating. While drilling ahead, the ECD from the circulating system is the pressure the wellbore wall and a short distance beyond experiences. Filter cake and any available sealing materials have been deposited at the borehole wall where there is permeability, thereby reducing the volume of mud filtrate into the wall. Any fluid in the fractures or between the bedding planes is now pressurized equivalent to the ECD. As a result of the ECD, the fractures are slightly dilated, as indicated by the heavy black lines. After the initial uptake of pressure, there are no changes in pressure that the wellbore hoop experiences until the ECD level changes to a new level. The hole appears stable while drilling.
Things can change dramatically with deliberate changes in wellbore pressure. When the mud pumps are stopped, as while making connections or during long trips out of the hole, the pressures in the microfractures and fractures equilibrate with the open system, and the bulk of the extra pressure returns to the wellbore given sufficient time. In short, the pressure takes the path of least resistance, a consequence of the shales’ permeability anisotropy. Admittedly, some of the pressure may dissipate into the far field.
In this scenario, the excess pressure that was in the fractures now equilibrates given enough time with the ESD. This change is primarily a high pressure/low volume pressure change. The release of excess pressure in the fractures causes the fractures to return to their native state, as indicated by the smaller black lines in Fig 6. This pressure reversal happens every time a connection is made, but the short time to make connections often masks the effect. On long trips out of the hole, especially after the first hour of tripping, the pressure reversals manifest themselves in terms of tight hole, packoffs, etc. If the ECD/ESD differential is kept within limits, the resulting ΔP will be fairly small. The magnitude of the pressure change can be calculated with Eq. 1 using the ECD/ESD differential value and TVD for a particular depth. On some ERD wells, the calculated ΔP can be as high as 1000 – 2000 psi.
With the pressure reversals, any weak rock at the wellbore wall is subject to increased pressure from behind by the ΔP as it pushes toward the wellbore. In Fig. 7, a portion of the laminated rock in Fig 1 is magnified with a circulating system superimposed on the side.
If any small pieces of weak rock are present at the wellbore wall, pressure reversals can dislodge them. Any piece falling into the wellbore creates a larger throat in the fracture opening that makes subsequent pressure and fluid invasion more facile. With time and more pressure fluctuations, other pieces begin falling, and the instability becomes more serious, as is drawn in Fig. 8.
One important consideration is that any piece of rock that is pushed into the wellbore will weaken further the fractured rock lying immediately above, as now the higher piece has lost its bottom support. Soon the scenario repeats and fractured rock begins peeling off the sides of the wellbore. With cavings accumulation reaching high levels in the wellbore (especially in the higher angled sections), pack-offs begin to occur, further exacerbating the pressure spikes that perturb the weaker laminated rock. These pressure spikes can be quite large in magnitude, as measured by downhole annular pressure tools.
As a consequence of the accumulation of cavings in the wellbore, the driller often takes action to better clean the wellbore, either by increasing pump rate or by backreaming the problem zone(s). In this proposed scenario, the problem will get worse as the ECD/ESD differential will increase, rather than decrease. In short, the wellbore stability deteriorates, and often increases in mud weight are seen as negative rather than positive. But it is neither the MW nor the ESD that is causing the instability problem, but rather the ECD/ESD differential.
In order to reduce the severity of the wellbore instability problem, some way must be found to reduce the ECD/ESD differential. Reductions in excessive flow rates and/or drill pipe rotation speeds can be investigated, along with secondary tools such as high-density sweeps to remove the larger-sized cavings from the wellbore. Ways have to be found to remove cuttings and cavings from the wellbore without the occurrence of more pressure spikes from pack-offs or use ofbackreaming as a cleaning tool. Hence effective wellbore pressure management (WPM) is necessary to successfully drill wells in these types of rock.
The following conclusions for understanding wellbore instability while drilling weak fractured or laminated shales or those with weak bedding planes can be offered:
- The instability in these wells is the result of pressure fluctuations in the wellbore. The greater the pressure fluctuations, the greater the severity of the problem.
- These pressure fluctuations are the result of pressure differentials between Equivalent Static Density (ESD) and Equivalent Circulating Density (ECD). Increases in mud weight, or ESD, are not the cause of the instability.
- Any pre-well wellbore stability modeling for these zones should employ the shale residual strength and not the shale intact strength. These shales may not lie at the interval TD, but rather may be further uphole and overlooked by researchers.
- The literature is filled with many conflicting causes and conclusions for wellbore instability in mechanically-weak zones. In the scenario presented in this paper, the mechanism is outlined and the consequences of overlooking the basic causes of the instability are demonstrated.
- Effective wellbore pressure management (WPM) is key to reducing the risk
associated with drilling in fractured or highly-laminated zones.