Dayna Rodriguez Zambrano

Drilling and Completion Strategies in Naturally Fractured Reservoirs

• Often, a natural fractured formation can be described as hard rock of low porosity and permeability.

Introduction

• Fracture planes will have a perpendicular direction to minimum in situ stresses (horizontal stresses).

• For that reason the most productive fractures will have subvertical planes (nearly vertical).

• The maximum in situ stress (σ1 in the graphic) represents the overburden.

Where:

σ1 >σ2 >σ3

Traditional Drilling Practices

In a Homogeneous Reservoirs:• Targeting the structural crest.• Manteining vertical borehole.• Over-balanced drilling.

In Naturally Fractured Reservoirs: • Targeting the structural crest may be

useless if the largest fractures are not reached.

• A vertical borehole will be parallel to the most productive fractures.

• Over-balanced drilling may cause loss of circulation, and possibly, permanent formation damage due to the penetration of drilling solids into the fractures.

• The well has to contact as many productive natural fractures as possible. This objective can be achieved by drilling a deviated well.

• Defining the fractures geometry.• Identifying fractured intervals with sufficient matrix porosity.• When the matrix rock permeability is low, locattion of natural

fractures become paramount.

Drilling Optimization - Drilling location

Drilling Optimization – Underbalanced Drilling

• Hidrostatic Pressure + Pumping Pressure < Formation Pressure

• High anisotropy: kv /kh is very high (kv is dominated by the fractures permeability and kh is dominated by the matrix permeability).

• Hydraulic fracture stimulation is a poor strategy in naturally fractured reservoirs because hydraulic fractures will propagate in the direction of the natural fractures without creating new ones.

Completion Optimization

• Geology.- The anticlinal structure was formed and dominated by wrench fault tectonics.

• The gas reserves in the Caranda Field are found in the Huamampampa and Roboré formations of Devonian age, and in the Sara sand of Silurian age.

• Because the deeper Roboré and Sara formations were the main exploration targets, the Huamampampa formation was logged, cased and cemented

Caranda Field - Bolivia

• The last completion design was not succesful.

• It was recommended drilling a sidetrack deviated through the Huamampampa formation.

• Underbalanced drilling was used.

• Roboré and Sara formations were completed with single production liner.

• Productive formations were cemented only above and below.

Colón Block - Venezuela

• The Colón Block is located in Venezuela southwest of Lake Maracaibo next to the border between Colombia and Venezuela.

• It was identified that the predominant structural feature in the Block to be the fault bend fold (FBF).

• Production data showed improved production rate and cumulative production from wells drilled on the limbs of the FBF structure. However, exceptions occurred in crestal wells.

• All of the productive fields and most of the exploration prospects have 2D seismic coverage.

• Borehole image logs provided the most reliable means for identifying and characterizing fractures.

• With matrix permeability ranging from less than 0.1 md to at best a few 10’s of md, significant production rate was a signal of the presence of natural fractures.

• Drilling at the top of the structural limbs was recommended.

• For a FBF structure with a N-S axis direction, drilling at an angle about 45 degrees from the N-S direction is recommended.

Conclusions

• An openhole completion could be recommended instead of cementing and perforating.

• In buckling structures, drilling the crest is recommended, but in FBF structures, the limb is preferred.

• Hydraulic fracturing in naturally fractured reservoirs is not recommended because the created fracture will propagate parallel to the open fractures in the reservoir instead of intersecting them.

• 3D seismic data is helpful for defining the structure configuration and setting the precise trajectory planning.

• In both studies, fracture identification from conventional logs was inconclusive, and borehole image logs are recommended as the direct way to identify the presence of natural fractures.