Well Productivity Evaluation and Implications for the Significance of Formation Damage: Oseberg Main Case History
Authors: Fleming Niall , Moldrheim Erlend , Teigland Espen , Mathisen Anne-Mette 
Published: February 2020

As part of an Equinor technical efficiency programme that was initiated in 2015 to deliver savings and improvements, bridging particles were removed from the drilling fluids of 15 wells on Oseberg Main and instead loss control material (LCM) was used, as required, in some but not all the wells. These long, horizontal wells were a combination of open hole (OH) and sand screens with and without inflow control together with cased & perforated (C&P) completions, producing from typical Brent Group sandstone formations with permeabilities varying from approximately 10 mD to Darcy sands, and which were depleted by as much as approximately 280 bars. In 2018, an extensive study was performed on these wells to determine the impact on inflow performance of drilling without bridging particles. It was realized that the 15 wells offered a worst-case scenario to study in the field rather than laboratory the significance of formation damage on well productivity. The data set generated offered a unique opportunity to challenge conventional formation damage assertions, especially for long, horizontal wells, examples of which are as follows.

Formation damage reduces productivity Significant mud losses with deep penetration lead to wells with lower than expected inflow performance Bridging particles are required to prevent formation damage around the near wellbore Drilling through heavily depleted reservoirs will result in severe formation damage Excessive use of graphite in an OH reservoir section will reduce inflow performance

The influence of different parameters, including loss control materials, lower completion design, loss type, mud penetration depth, dynamic overbalance while drilling, length of production interval, net to gross (NTG) and kh were considered for those wells drilled without bridging particles. One of the surprising findings was that there was no clear evidence that losses were detrimental to the productivity of these long horizontal wells, i.e. it would appear that the Brent reservoir sections, despite being depleted, were more resistant to the influence of formation damage on inflow performance than first thought. Furthermore, for this example bridging particles appear to be of less importance in the avoidance of formation damage but are important in preventing excessive increases in fluid costs due to losses.

After a thorough review of all the data obtained from this study, together with the conclusions drawn, it was realized that these had direct implications for Equinor's approach to fluid qualification, and especially coreflooding. The most important conclusion that influenced this change in approach was that the long reservoir sections (approximately 1 km or more) within typical Brent heterogeneous formations appear to tolerate more formation damage without impairing the productivity index (PI). A direct consequence of this was the conclusion that more emphasis should be placed on fluid compatibility, mobility, screen plugging and stability along with particle size distribution (PSD) design while the importance of coreflooding to fluid qualification was downgraded for Brent and reservoirs of similar chacteristics. This is not to say that coreflooding will not be performed, but rather it will be targeted towards situations where the influence of formation damage on well productivity is more significant, e.g. HTHP fields where special drilling and completion fluids are required, low permeable formations without mechanical stimulation, and shallow reservoirs with low reservoir temperature.

This paper will perform an evaluation of the significance of formation damage on well productivity and use this to demonstrate Equinor's revised approach to formation damage laboratory evaluation based upon field experiences.