Judy Field – Rejuvenation Through A Second Phase Of Drilling

The Judy Field is a northwest plunging horst located on the northern part of the Josephine Ridge in Block 30/7a, UK Central North Sea. The field is subdivided by a series of north-northwest and northwest striking faults which form isolated fault blocks. The 30/7a-11z well was the fifth appraisal well in Judy Field and is located in a fault block to the north of the Judy Platform. The primary producing reservoirs of this field are the Middle to Upper Triassic Joanne and Judy Sandstone Members of the Skaggerak Formation with minor contribution from the Upper Jurassic Fulmar Formation. Detailed analysis of the Triassic core from the appraisal wells revealed 11 facies types which were grouped in four main facies associations (channel sandstone and sheetflood sandstone as reservoirs and argillaceous sandstone and lacustrine mudstone generally as non-reservoirs). The present-day porosity and permeability of these facies is the result of the relatively high primary porosity being slightly enhanced by diagenesis. The main diagenetic enhancement mechanisms were carbonate and feldspar dissolution by aggressive pore fluids. Predicting the reservoir performance from the Triassic section is difficult since the core data indicates a highly permeable reservoir whereas the interpreted depositional environment suggests a much lower interconnected permeability. Initial drill stem tests from the appraisal wells indicated horizontal permeability (Kh) values approximately one order of a magnitude lower than the core data. Based on these data, a simplistic layered reservoir model was created to mimic the relative variability in Kh seen in the core but with absolute values closer to the DST derived Kh. This model was utilised to site seven production wells which targeted the main interpreted fault blocks in the field. The long-term production from these wells indicated that the 'effective permeability' of the reservoir was another approximate order of magnitude lower than the DST Kh data. The lower than anticipated permeability resulted in a more rapid decline from plateau rate than anticipated but the anticipated reserves would be produced over a longer period of time.