With the Schlumberger Sand Management Advisor sand-prediction model, critical draw- down pressures were calculated across the anticipated completion intervals for three differ- ent horizontal completion scenarios: openhole completion, horizontal perforations and vertical perforations (previous page, top). Results indi- cated that resistance to sand failure dramatically improves for perforated completions and sand production could be delayed for several years. Additionally, perforations oriented vertically were significantly more stable than those oriented horizontally.
The critical drawdown analysis showed that under openhole conditions, the reservoir sands could withstand a drawdown of 2,175 psi [15 MPa] at the initial reservoir pressure of 4,550 psi [31.4 MPa]. However, simulation pre- dictions for the fi eld-development plan indicated that formation pressures would deplete to the extent that any amount of drawdown would cause sand failure after one year of production. Searching for an alternative to sand screens, which pose a high cost of installation and a risk of failure in later life, BP explored perforated completion options.
Horizontal perforations— the worst of the cased-hole perforation scenarios— would improve the resistance to failure compared with the openhole case, allowing drawdown to 3,150 psi [21.7 MPa] at the initial reservoir pres- sure. After four and a half years of production, once the reservoir pressure dropped below 1,350 psi [9.3 MPa], sand failure would be expected. When perforations are oriented in the more stable vertical direction, the 3D sand- prediction model suggests that the formation should not fail until the reservoir pressure declines to less than 500 psi [3.4 MPa], which is beyond the anticipated economic life of the fi eld. The most stable perforation orientation is actu- ally angled slightly away from vertical to increase the distance between perforations, thereby reducing the overlap of stress concentrations around each perforation.
The critical drawdown analysis was extended to include the entire completion interval to examine the long-term effects of reservoir deple- tion (previous page, bottom). The three scenarios were again examined and compared at the ini- tial reservoir pressure, at 2,000-psi [13.8-MPa] depletion for the openhole case, and at 3,000-psi [20.7-MPa] depletion for both the horizontal and vertical perforation scenarios. The weakest sands and shaly intervals within the section were predicted to fail regardless of perforation orientation, so the completion
design recommended that engineers avoid perfo- rating them.
The East well was drilled and completed in 2002. Critical drawdown pressures were recalcu- lated using the East well log data, which included density and tc and ts from an LWD sonic tool. A multidisciplinary team selected perfora- tion intervals from the revised analysis, and the oriented perforating job was completed success- fully. Later in 2002, the same procedure was followed on the West well. The wells were put on production in November 2002 and exhibited slightly negative skin values, indicating no forma- tion damage. With the exception of some initial
fi ne-grained sand production resulting from per- foration cleanup, the wells are producing with only minimal sand infl ux, matching expectations from the modeling.
Going Screenless in Japan Amarume fi eld, operated by the Japan Petroleum Exploration Company, Limited (JAPEX), is an onshore oil field close to Niigata on Honshu Island, Japan (right). Produced since the early 1960s, the fi eld has a reservoir pressure that has dropped from 1,800 psi [12.4 MPa] to 650 psi [4.5 MPa]. Sand production observed in six wells led JAPEX to reduce drawdown in all the wells to 55 psi [0.4 MPa].
During the planning stages of Well SK 74D, Schlumberger proposed a completion solution that would help limit sand production. Prior to selecting the type of completion, the planning team characterized reservoir stresses and completed a sand-prediction analysis. Special software was used to translate the
gamma ray, density and sonic— tc and ts— data from a vertical offset well, the AMR TRC-1, to the planned deviated SK 74D well. RockSolid wellbore stability software was then used to construct a 1D MEM, which was calibrated with core-strength data from multistage triaxial tests. Borehole images were not available, but because the zone of interest was shallow, it was thought that the orthogonal horizontal stresses were sim- ilar in magnitude. However, a previously acquired vertical seismic profi le (VSP) and the DataFRAC pressure data indicated stress anisotropy did exist.
The Sand Management Advisor sand-
prediction software, using the 1D MEM as input, provided the critical drawdown pressures to iden- tify potential sand-production zones. A combination of low unconfined compressive strength (UCS) between 100 and 500 psi [0.7 and 3.4 MPa] and 60% reservoir depletion meant that shear failure could occur at the sand- face in the perforations. During the analysis,
Spring 2004
0 0
km
Amarume oil field
Niigata J A P A N
200 miles 200
> Location of Amarume eld in Japan.
perforated completion parameters were examined, including perforation deviation from vertical, perfo- ration diameter and perforation orientation. Plots show that sand failure would occur, regardless of the perforating job design. The critical drawdown plots predicted sand production at any drawdown pressure and indicated a need for sand control. The low reservoir pressure and predicted low well pro- ductivity made any sand-exclusion screen completion uneconomical and instead suggested the need for a less expensive, screenless hydrauli- cally fractured completion.
For a screenless fractured completion in this well, the following steps were identifi ed as critical to treatment success:
• High-quality well cementing ensures good zonal isolation across weak and depleted zones. The success of any fracturing treatment requires good zonal isolation to help pump fracturing fluid and proppant to the desired zone. Achieving zonal isolation is even more of a challenge when cementing across depleted reservoirs where loss of circulation is antici- pated. For this well, a two-stage cementing operation with 1.6-specifi c gravity LiteCRETE slurry was used.28
Cement bond logs and USI
UltraSonic Imager logs indicated good bonding over the cemented intervals.
• Optimized perforating helps ensure that for- mation fluid is filtered through the proppant
27. Ali et al, reference 17.
28. Al-Suwaidi A, Hun C, Bustillos J, Guillot D, Rondeau J, Vigneaux P, Helou H, Martí nez Ramí rez JA and Resé ndiz Robles JL: “ Light as a Feather, Hard as a Rock,” Oil eld Rev iew 13, no. 2 (Summer 2001): 2– 15.
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