Mud Losses in Fractured

Carbonate Formations

John Emeka Udegbunam, University

of Stavanger, Norway

Carbonate Reservoirs

• Sedimentary rocks made of carbonate minerals

– limestone, dolomite and chalk

• Hold > 60 % oil and 40 % gas reserves globally

• Middle East accounts for 70 % oil and 90 % gas

reserves [1]

• Most of these carbonates are naturally fractured

• Fracture pathways can be beneficial during HC

production

• Mud losses is common during drilling

Naturally Fractured Reservoirs

Figure 1: A naturally fractured reservoir (left) and its model (right) [3]

Naturally Fractured Reservoirs

• Have a continuous network of well-distributed

fractures

• Absence of fluid transition zones

• Constant PVT properties with depth

• Low producing GOR

• Low pressure drop around producing well

• Presence of fracture network gas cap [2]

Naturally Fractured Reservoirs

(a) Conventional reservoir (b) Fractured reservoir

Figure 2: Fluid transition zones in a non-fractured carbonate reservoir [2]. There is absence of transition zone in a naturally fractured reservoir due to negligible capillary effects.

Fracture Identification

• Significant mud loss while drilling

• Lab core examination

• Observation of outcrops

• Special behaviour in transient pressure analysis

• Use of televiewer (logging)

• Seismic evaluations [2]

Fracture Network

• A single fracture (defined by width, size and

nature)

• Fracture system: a set of parallel fractures

- can align horizontally or vertically

• Fracture network: two or more linked fracture

system [2]

Fracture Network

Figure 3: Natural Fracture network [4]

Group Fracture Parameters

Figure 4: Fracture systems

• Linear fracture density

- (LFDV/LFDX;

LFDH/LFDZ)

• Fracture intensity (FINT)

Wettability and Capillary Pressure

Figure 5: Wettability measurement for water-oil-solid system [2]

• Wetting phase

• Non-wetting phase

• Contact angle (θ)

• Capillary pressure

Pc = Pnw – Pw = Po – Pw

Wetting index (cosθ)

Contact angle

(θ)

Wetting condition

1.0 0° Completely water-wet

0 90° Neutral wet

-1.0 180° Completely oil-wet

Table 1: Wetting indices for different wetting conditions

Lost Circulation in Fractured Reservoirs

• High tendency for drilling fluid losses

• Causes kick, blowout

• Well control problem

• Invasive formation damage

Effects of Drilling Fluid Section and Reservoir Wettability

• Drilling fluid (mud) - OBM, WBM & SBM

• Water-wet reservoir vs WBM

• Oil-wet reservoir vs OBM

• The liquid phase will imbibe in the matrix

- additional mud loss

Measures to Reduce Lost Circulation

• Proper drilling fluid selection

• Proper fracture identification

• Use of LCMs

• Application of underbalanced operations

• Use of advanced drilling methods- MPD & DGD

Conclusion

• Mud engineers should consider wetting conditions of fractured carbonate reservoirs

References 1. Schlumberger, 2012: “Carbonate Reservoirs,” slb website.

2. Torsæter, O., 2012: “Reservoir Engineering of Naturally Fractured Reservoir”, Norwegian University of Science and Technology (NTNU), Trondheim, Norway.

3. Warren, J. E., and Root, P. J., 1963: “The Behavior of Naturally Fractured Reservoirs”, SPE Journal of Petroleum Technology, 426-PA, September, 245-255.

4. Narr, W., Schechter, D. W., and Thompson, L. B., 2006: “Naturally Fractured Reservoir Characterization”, Text book Series, SPE, Richardson, Texas, USA.