Development of Frac-and-Pack Techniques in a Multilayer Reservoir, Offshore Italy
Authors: Maroli R. , Ripa G. , Menilli G. , McCulloch D. 
Published: December 2007

A major workover program of the Emma Field offshore Italy was initiated to increase the number of draining points in the main body of a multilayer reservoir, which would in turn, increase the field production levels. Emma is a gas-producing field located in the Adriatic Sea, offshore central Italy. The field is operated by AGIP (51%) in association with Edison Gas (49%). The main sand body of the reservoir was recompleted in up to 10 intervals with a series of production packers, with each layer being gravel-packed or frac-and-packed to prevent the production of formation sand.

The paper will review the effectiveness of the completion techniques applied to thin producing layers of this multilayer reservoir; in particular, the paper will evaluate the various sand control techniques applied throughout the project. These developments included the use of various density brine-based fluids and a relatively low-viscosity, sheared and filtered 40-lb/Mgal gel as the gravel carrying medium.

A total of 12 frac-and-packs were completed in the primary producing layers; the proppant amounts placed in the induced fractures varied from 3,000 to 22,000 lb. This placement resulted in simulated proppant concentrations in the induced fractures from 1 to 8 lb/ft2. An industry-standard real-time fracturing design simulator predicted and monitored the fracture geometry created.

Many of the treatments were performed with the gravelpack tools in a circulating position, so that a live annulus pressure was available. This live annulus pressure allowed the monitoring of the real-time bottomhole pressure and its use in the net fracturing pressure analysis.

We were also able to examine the effect of varying pad volumes in the same intervals on different wells, and observe how variances affected the net pressure responses in this very low Young's modulus sandstone of 3E5 psi.

Finally, an evaluation of the various methods of completion and sand-control mechanisms is supported by production history data.

Introduction

Emma is a gas-producing field located 30 km offshore central Italy in the Adriatic Sea. This field was discovered in 1971. During the exploration phase, test results showed that the main gas-bearing reservoir extended from 1250 to 2080 m across 20 separated levels. Based on the positive results obtained in two exploration wells, eight wells were drilled and completed with two selective strings between 1981 and 1983. These wells were completed from a platform in a 100-m water depth. After 10 years of production, a new well was drilled in 1993 to verify the potential productivity of thin silty-shale layers. Based on the results of the production test, a major workover program was planned to develop these new levels and increase the number of drainage points in the main body of the multilayered reservoir, which would increase overall gas recovery from the Emma field.

Reservoir Description

Emma field is a NW-SE anticline with an areal extent of 6 km2. The multilayer gas-bearing Pliocene formation is characterized by a sequence of sand strata interbedded with shale. In the first phase (1982 through 1992), only 12 of the 20 levels were completed and they produced without sand problems.

The average porosity value was 25% and the average water saturation was 45%. These values were not constant in the reservoir, but were related to the depth of the level, the well location, and the different shale content of each level. The pay- zone thickness ranged from 1 to 40 m, with formation gas permeability from 8 to 40 md. Initial reservoir pressure was 228 kg/cm2 at 2080 m with a BHST equal to 51.5 C.

Because of the production decline observed during the first 10 years and based on the encouraging results obtained from thin layers, a workover was planned to complete these thin layers and increase ultimate gas recovery.

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