Database
[{"_id":ObjectId("612ff22c17286411a17252cf"),"WELL":"199-H4-15A","TYPE":"E","SYSTEM":"HX","ID":"199-H4-15A_E_HX","Jan-14":-168.8,"Feb-14":-151,"Mar-14":-164.1,"Apr-14":-168.7,"May-14":-172.6,"Jun-14":-177.3,"Jul-14":-177.6,"Aug-14":-171.9,"Sep-14":-138.9,"Oct-14":-130.3,"Nov-14":-163.8,"Dec-14":-161.4,"Jan-15":-168.7,"Feb-15":-168.9,"Mar-15":-168.6,"Apr-15":-164.6,"May-15":-141.7,"Jun-15":-153.5,"Jul-15":-163.7,"Aug-15":-167.7}]
Query
db.collection.aggregate([{$match:{"_id":ObjectId("612ff22c17286411a17252cf")}},{"$addFields":{"data":{"$objectToArray":"$$ROOT"}}},{"$addFields":{"data":{"$filter":{"input":"$data","as":"tuple","cond":{"$isNumber":"$$tuple.v"}}}}},{"$addFields":{"data":{$avg:"$data.v"}}}])