drilling and completion strategies in naturally fractured reservoirs
TRANSCRIPT
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.